How to Make Sure Your Fish Tank Is Level
DISCLAIMER: THIS IS MY PERSONAL OPINION AS A FORUM MEMBER AND DOES NOT REFLECT NECESSARILY THE OPINIONS OF THE MODERATOR OR OTHERS HERE. It is based upon MY years of keeping tropical fish.
Tips and Concern about Leveling your Aquarium
Often times people bring home a new aquarium and place it where they want it and check it with a level before filling it with water. Thats all good and they should as a beginning point. The bubble of the level should be relatively close to middle and in between the two little lines. But if it's not 100% centered there is no need to panic. Consider that some people are placing their aquariums directly on top of a carpeted floor etc. and that the aquarium is thus far empty. Once you fill that aquarium, things have a way of leveling out because all that extra weight compresses the carpeting / flooring material and you are probably good to go. So, if the level is close, you should be fine. Once filled halfway or so with water, you should have no more than 1/8 of an inch and certainly no more than 1/4" difference at each end of your Aquarium, measuring now from the surface
Rule #1 is that water seeks its own level and in fact will always be level.
Lastly, consider that any shims you do use will see tremendous crushing force PSI (pounds per square inch) and thus must be strong enough to sustain the point pressures of all that weight. (Foam is not an acceptable shim as that acts only as padding and easily compresses under the waters weight to where you started with in the first place.) To illustrate my point, take a piece of foam and place it on the floor and then stand on it. See if it holds you up or if it simply flattens out under the pressure and your weight. (In aquarium use, foam is a "feel good" material because it makes you feel good about using it but it does absolutely nothing.) In general,Plastic too should not be used for leveling a stand as most plastic can (and will) later crack or break down. Plastic may hold for a while but when you least expect it, it could fall apart later on which could then leave you in a precarious situation. Shims should only be solid wood or metal capable of sustaining the point weight pressures they will see. In general, I have found that as long as your level is between the two lines, you should be OK once it settles in. If your aquarium is off a bit because your floor is off a bit, you are probably better off without shims at all as long as the level from end to end is less then 1/4" off, especially if you're talking about a 4 foot or longer aquarium. Just my opinion from my experience...
Note: Most aquarium specific stands have a flat bottom without legs. That is certainly easier to shim then a stand of some sort that has legs only on the corners or worse yet, 6 or more individual legs. Extreme care must be taken to shim a stand with legs and you must ensure that the stand remains solidly on the ground with no "wobble". Many people today buy "tall" aquariums and those present another factor. It is vitally important to check the level front to back (especially with tall tanks) as they can easily lean forwards or backward and could fall over with enough coaxing from children, pets or general foot traffic. You might consider anchoring the stand to a wall to prevent this situation.
Article by Ken T.