Depends what you want to qt. It's not a huge diff between them. If you need to treat, smaller volume may helpI bought a second hand 35 gallon tank that I was going to use for the sump. It fit, barely, but there wasn't enough room to get in there and redo the plumbing or do any work on it for that matter so I scratched that idea and found a 30 gallon used tank that has a little more room to work on it. Now the plan is to use the 35g for baffles in the 30g.
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Here's the 35g that I was going to use for the sump. You can see that it's too tall to do any work on it once it's installed in the stand.
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Here's the 30g that I found to use for the sump. This one will give me more room to work. The aluminum stand that it's sitting on is going be used for my QT. I've got a 45g and a 55g and I'm thinking of using one of those for the QT. Which one does everyone think I should use or should I go smaller?