“Best” needs a job to do
A fish can be your strongest current income source, a useful temporary slot, or a poor fit for the upgrade you want next. Decide which question you are answering before comparing anything.
Record only what you can verify
Read the fish label and its displayed income in your own game. Keep the quantity separate from income per fish. If a value cannot be checked, leave it out rather than turning an estimate into a fact.
Measure contribution to the whole aquarium
A single fish value does not show how much a group contributes. Add each label, quantity, and per-fish income to the aquarium income calculator. The ranked output reveals which group currently carries the largest share.
Compare the change, not just the new total
When considering a replacement, compare the aquarium before and after the change. The difference tells you the actual income gain. If the change requires an upgrade, enter that gain and the cost in the payback calculator.
Recheck after every meaningful change
Your answer changes with your fish mix and goals. Run your latest aquarium numbers again after reorganizing, then keep the fish that best supports the decision in front of you.