If you have the money and equipment ,,,, I say at least a 75..... I actually think that a bigger tank is e-z'er to take care of than a small one,,,, in a small tank if something dies the levels can jump up way fast causing harm to the others,,,, and in a bigger tank you can add a bit more rock, critters,ect.... that will help each other out,,,, your own ocean system.....