I have a 125 gallon tank. I'm only going to use 2/3rd of the tank. That's plenty for one oscar. That's roughly 75-80 gallons.

I realize the importance of an absolutely level tank. Problem is everytime I shim it and fill it with water--THOUGH IN BETWEEN THE TWO LINES OF THE LEVELER, I can't make it absolutely perfect. When I make one area perfect it slightly offsets the other!

I have these plastic shims under the tank STAND (supposed to be stronger than hardwood) and it's nearly in the center of the leveler but still between the two lines.

Is this acceptable? I really tried to shim it the best I could. I mean if it's just a hair off exact center (and it's impossible to make it perfect), would this be enough to cause a stress fracture??

Thanks

Your situation sounds fine. My 125 gallon 6 x 2 x 2 is not exactly level and it's been sitting there for quite a few years now without any trouble. Levelling is just a precaution, I don't know of anyone whose tank has cracked because of this problem. There must be someone out there or we wouldn't be addressing this problem. Sometimes you just can't get it perfect. As long as it isn't too bad, I think you should be okay.

I think there is something that we must address when setting up an aquarium. This really only applies to very large aquariums. If you are setting up a wooden floor though it's obvious that you put the aquarium somewhere extremely safe. Preferably against a supporting wall. Most large aquariums that come with a Cabinet don't have any feet. They are completely flat. There is a very good reason for this, it dissipates the weight evenly over the floor. If you start putting in feet, that is going to dig into the floor quite badly. Just bear that in mind when you are shimming

You are worrying about the two lines of the level when you SHOULD be much more concerned about how far off each end is once you have it partially filled with water.

Measure at each end of the tank from the waters surface to the top rim of the tank.
Those two should be within a 1/4" of each other. If they are, they you are ok. If the difference there is more then 1/4" (say 3/8 -1/2") then I would remove the existing water and re-shim accordingly. Just remember, if your tank is 6 foot in length and you need to shim for a 1/4" difference end to end, then the shim you want to use is not 1/4" but is really going to be much much thinner, because over the 6' length it's going to make quite a difference..

(You use the level on an empty tank as a starting point.)

Penn is correct. an unlevel tank does not mean it will be caused to crack or stress (unless greatly off level) but a tank that gets shimmed in the wrong place (between the tank itself and the stand) will see high and low stress points of contact and is much more likely to fail. Always shim under the stand, between the stand and floor.

Yeah, never put anything hard between the tank and the stand, that would be disastrous. Always give it a good dusting down as well. Even the tiniest bit of debris could crack the tank. On a couple of my tanks, we have put very soft material between the tank and the cabin that just to be on the safe side.

This is very reassuring! Thank you so much for your input. My cousin did have this huge 150 gallon aquarium but it did break--but I think it had more to do with the KIND of fish--he had some really huge saltwater fish and they most likely were ramming against the glass. They can ram pretty hard too.

I'm lucky my previous oscar didn't break the glass of my previous 55 gallon tank. A few instances he woke me up in the middle of the night because he rammed so hard against the glass; with him 14 inches long it most likely the fish tank was too small for him.

This oscar I have does not appear aggressive and is very well mannered. So maybe I lucked out plus the 125 gallon tank he should be happy!