Semi-Aggressive Fish Compatibility Chart
Semi-aggressive fish compatibility is one of the trickiest areas of freshwater fishkeeping to get right. Semi-aggressive fish fall into a gray zone — they are not peaceful enough for a standard community tank, but they are not outright predators either. We have spent years figuring out which semi-aggressive species can live together and which combinations end in disaster. This guide is the result of that experience.
Whether you keep Oscar fish, jack Dempseys, convict cichlids, or other semi-aggressive species, this compatibility chart and guide will help you stock your tank with confidence.
What Does Semi-Aggressive Mean?
Before we get into specific combinations, it is important to understand what “semi-aggressive” actually means in fishkeeping. It is a broad label that covers a wide range of behaviors, and not all semi-aggressive fish are created equal.
Definition and Range
Semi-aggressive fish are species that show territorial or combative behavior under certain conditions but are not constant aggressors. They may fight during breeding, defend a specific area of the tank, or become hostile toward fish that look similar to them. Outside of these triggers, many semi-aggressive fish are relatively calm. The label covers everything from mildly pushy fish like gouramis to borderline aggressive species like Oscars and jack Dempseys.
Factors That Affect Aggression
The same fish can be peaceful in one tank and aggressive in another. Tank size is the biggest factor — cramped conditions trigger territorial behavior in almost every species. Other factors include: breeding status, sex ratio, number of hiding spots, feeding schedule, water quality, and individual personality. Understanding these variables helps you predict and manage aggression before it becomes a problem.
Categories Within Semi-Aggressive
We find it helpful to break semi-aggressive fish into three sub-categories: mildly semi-aggressive (gouramis, rainbow sharks, tiger barbs), moderately semi-aggressive (angelfish, firemouths, severums), and strongly semi-aggressive (Oscars, jack Dempseys, green terrors). Fish within the same sub-category tend to be the most compatible with each other. Mixing sub-categories requires careful planning and larger tanks.
The Compatibility Chart
Here is our master compatibility chart for the most popular semi-aggressive freshwater fish. We have based this on our own experience and the collective experience of the fishkeeping community. Remember that individual results may vary — these are general guidelines, not guarantees.
| Species | Oscar | Jack Dempsey | Firemouth | Convict | Severum | Angelfish | Green Terror | Rainbow Shark |
|---|---|---|---|---|---|---|---|---|
| Oscar | Caution | Yes (150g+) | Maybe (125g+) | No | Yes (125g+) | No | Maybe (200g+) | No |
| Jack Dempsey | Yes (150g+) | Caution | Maybe (75g+) | Maybe (75g+) | Yes (75g+) | No | Maybe (125g+) | No |
| Firemouth | Maybe (125g+) | Maybe (75g+) | Caution | Maybe (55g+) | Yes (75g+) | Maybe (75g+) | No | Maybe (55g+) |
| Convict | No | Maybe (75g+) | Maybe (55g+) | Caution | Maybe (75g+) | No | No | No |
| Severum | Yes (125g+) | Yes (75g+) | Yes (75g+) | Maybe (75g+) | Caution | Maybe (75g+) | Maybe (125g+) | Maybe (55g+) |
| Angelfish | No | No | Maybe (75g+) | No | Maybe (75g+) | Caution | No | No |
| Green Terror | Maybe (200g+) | Maybe (125g+) | No | No | Maybe (125g+) | No | Caution | No |
| Rainbow Shark | No | No | Maybe (55g+) | No | Maybe (55g+) | No | No | No (1 only) |
Key: Yes = generally compatible at the stated minimum tank size. Maybe = can work with the right setup and monitoring. No = not recommended. Caution = same species pairing requires specific conditions.
How to Read the Chart
Find one species on the left column and the other on the top row. The cell where they meet gives you our recommendation. The gallon number in parentheses is the minimum tank size we recommend for that specific combination. If it says “No,” that combination is too risky regardless of tank size. If it says “Caution,” it means keeping multiples of the same species requires careful planning — usually you need either a bonded pair or a large group to distribute aggression.
Why Some Popular Combinations Say “No”
You might be surprised that Oscars and convict cichlids get a “No.” Convicts are small but incredibly aggressive, especially when breeding. They will harass an Oscar relentlessly, and while the Oscar could eat them, convicts are fast and persistent. Similarly, Oscars and angelfish get a “No” because the size difference makes it too dangerous for the angelfish. We cover this specific combination in detail in our Oscar fish tank mates guide.
The “Maybe” Combinations
“Maybe” is the most common rating because semi-aggressive fish compatibility depends heavily on individual circumstances. These combinations can work but require: the recommended minimum tank size (or larger), plenty of hiding spots and visual barriers, close monitoring for the first few weeks, a backup plan if things go wrong, and willingness to separate fish if needed.
Best Practices for Mixed Semi-Aggressive Tanks
If you are going to mix semi-aggressive species, there are strategies that significantly improve your chances of success. We use all of these in our own tanks.
Always Oversize the Tank
Whatever minimum tank size you see recommended, add 25-50% more volume if possible. Extra space is the single best aggression reducer available to you. In a cramped tank, even compatible species will fight over territory. In an oversized tank, even borderline combinations can work because each fish can establish its own space without constant conflict. For Oscar-specific tank sizing, see our tank setup guide.
Create Distinct Territories
Use large rocks, driftwood, and sturdy decorations to divide the tank into multiple distinct areas. Each semi-aggressive fish needs a home base that it can call its own. When territories are clearly defined by physical structures, fish spend less energy defending borders because the boundaries are obvious. Think of it like putting walls between apartments rather than having one big open room.
Stock All Fish Simultaneously
Whenever possible, introduce all fish to the tank at the same time. This prevents any single fish from establishing dominant ownership of the entire tank before the others arrive. If you must add fish later, rearrange all the decorations first to reset territorial boundaries. Adding a new fish to an established tank is one of the highest-risk moments for aggression.
Common Semi-Aggressive Species Profiles
To make the best stocking decisions, you need to understand each species individually. Here are profiles of the most popular semi-aggressive freshwater fish.
Oscar Fish
Oscars are the kings of the semi-aggressive category. Growing to 12-14 inches, they are the largest commonly kept semi-aggressive fish. They are intelligent, personable, and can be quite gentle for their size — but they will eat anything that fits in their mouth and they need a lot of space. Best tank mates are similarly sized, robust fish. Learn about their different varieties in our Oscar fish types and Oscar fish colors guides.
Jack Dempsey Cichlid
Named after the famous boxer for good reason, jack Dempseys are 8-10 inch cichlids with a reputation for toughness. They are actually quite compatible with Oscars in large tanks because they are big enough to avoid being eaten and tough enough to hold their own without being constant aggressors. They claim a specific cave or rock as their home base and mostly leave others alone outside that zone.
Severum
Severums are sometimes called “poor man’s discus” because of their round body shape. Growing to 8-10 inches, they are one of the more peaceful semi-aggressive cichlids. They are herbivorous-leaning omnivores that rarely start fights. Severums make excellent tank mates for both Oscars and jack Dempseys because they are large enough to coexist but mellow enough not to cause problems.
| Species | Max Size | Min Tank (Solo) | Aggression Level | Best With |
|---|---|---|---|---|
| Oscar | 12-14″ | 75 gal | Strong semi-aggressive | JD, Severum, Silver Dollar, Pleco |
| Jack Dempsey | 8-10″ | 55 gal | Strong semi-aggressive | Oscar, Severum, Firemouth |
| Firemouth | 6-7″ | 30 gal | Moderate semi-aggressive | Severum, JD, Angelfish |
| Convict | 4-5″ | 30 gal | Strong semi-aggressive | Similar-sized cichlids only |
| Severum | 8-10″ | 55 gal | Mild semi-aggressive | Oscar, JD, Firemouth, Angels |
| Angelfish | 6″ body | 30 gal | Mild semi-aggressive | Severum, Firemouth, community |
| Green Terror | 8-10″ | 55 gal | Strong semi-aggressive | Oscar (huge tank), JD |
| Rainbow Shark | 6″ | 55 gal | Mild semi-aggressive | Firemouth, Severum |
Mistakes to Avoid
We have made most of these mistakes ourselves over the years, and we want to help you avoid them.
Mixing Drastically Different Size Classes
A 12-inch Oscar and a 3-inch convict cichlid are technically both semi-aggressive, but the size difference makes them completely incompatible. Larger fish will eat smaller ones, full stop. When mixing species, try to keep all fish within a factor of two in size — a 10-inch Oscar and a 6-inch severum is fine; a 10-inch Oscar and a 2-inch ram is not.
Ignoring Breeding Aggression
Many semi-aggressive fish are manageable until they breed. Breeding convicts, jack Dempseys, and green terrors become extremely aggressive and will attack everything in the tank. If you keep mixed-sex groups of these species, be prepared for breeding-related aggression. Either keep same-sex groups or have a plan for separating breeding pairs. Our breeding Oscar fish guide covers this in detail for Oscars.
Relying on Pet Store Advice
We say this with respect for good fish stores, but many pet store employees lack specific experience with semi-aggressive fish compatibility. They may suggest combinations based on general labels without considering size, temperament, or tank requirements. Do your own research, consult experienced fishkeepers, and always have a separation plan before mixing semi-aggressive species.
Frequently Asked Questions
What fish can live with semi-aggressive fish?
The best fish to keep with semi-aggressive species are other semi-aggressive fish of similar size and temperament. Dither fish like silver dollars and giant danios can also work well — they are fast, schooling fish that help diffuse aggression by keeping the semi-aggressive fish focused on the school’s movement rather than on individual targets. Large, armored catfish like plecos and synodontis are also safe choices because they are too tough and too uninteresting for most semi-aggressive fish to bother.
Can you keep two different semi-aggressive cichlids together?
Yes, many semi-aggressive cichlid combinations work well in the right conditions. The keys are: matching size classes (fish should be within 2-3 inches of each other), providing enough tank space (typically 75 gallons minimum for two large cichlids), and creating distinct territories with decorations. Some of the best pairings include Oscars with jack Dempseys, severums with firemouths, and jack Dempseys with severums.
How big should a tank be for semi-aggressive fish?
Tank size depends on the specific species, but as a general rule, plan for at least 55 gallons for a single semi-aggressive cichlid and 75-125 gallons for a community of mixed semi-aggressive fish. For large species like Oscars and green terrors, start at 75 gallons for one fish and add 50-75 gallons for each additional large cichlid. More space always reduces aggression. Our tank setup guide has specific recommendations.
Do semi-aggressive fish need hiding spots?
Absolutely. Hiding spots are critical in semi-aggressive community tanks. Every fish needs at least one retreat where it can escape from aggression, and having multiple hiding spots throughout the tank reduces territorial fighting. Use caves, large rocks, driftwood, and dense plant arrangements to create sheltered areas. Without these retreats, subordinate fish have nowhere to go when chased, which leads to chronic stress and potential death.
Is it better to keep semi-aggressive fish alone?
Keeping a single semi-aggressive fish is often the simplest and safest option, especially for beginners. A solo Oscar, jack Dempsey, or severum will be perfectly happy on its own and will not suffer from loneliness. The main advantage of keeping them alone is zero aggression risk and easier tank maintenance. Community tanks with multiple semi-aggressive fish are rewarding but require more experience, larger tanks, and constant monitoring.
Last Updated: March 15, 2026
Written by the team at OscarFishLover.com. We are passionate fishkeepers with years of hands-on experience raising Oscars and other freshwater species. Learn more about us on our About page.