Eventually yes, an overstocked tank will kill the fish. Poor water quality is likely which is what would kill the fish. Depending on the tank and the fish, the tank could also stunt the fishes growth killing it.

Edit-
Correct, depending on the type of sucker fish you have it will likely need a larger tank. Plecos and chinese algae eaters get huge but are often sold to owners with tanks too small. Danials? Do you mean danios by chance? If not those require longer wider tanks as they are active fish.

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