I assume by "suckers" you mean plecos? Or maybe Siamese or Chinese Algae Eaters? I can't help much without knowing the species

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Why doesn't the tank have a heater? It's not so much the cooler temperatures that will bother the fish but rather the fact that the temperature keeps changing. I keep my tropical tanks at a steady 78 degrees.
Has the water been tested? I would test ammonia, nitrites, and pH especially. If the pH is insanely low it could be from high nitrates, btw.
Also, please remember that even "suckers" can't live on just algae. They'll slowly starve to death. You need to supplement their diet with frozen foods (such as bloodworms), algae wafers, and vegetables.
What size tank is it? Plecos need a TON of space and good filteration.