It's not good idea to buy tanks that look like pyramide. Why? Because the surface area is important when water is absorbting O2 and releases CO2 to air. When there isn't enough surface area, fishes will suffer low oxygen level and die eventually.
If the pyramide is big enough and has large surface area (so you cannot fill it, tank, full, then it's ok
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Not good idea:
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Better idea:
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