Your pictures make me think ich, because these are goldfish, use medicine, DO NOT raise to 86f to kill.
If parts of the fins are missing (cant tell from the pics) fin rot sounds more likely.
If unsure can purchase a general tank medicine. If you go this route also buy something like seachem stability to replenish your nitrogen cycle.
Do a rather large water change like 50% to reduce the bio load in the water. The. Treat with medication. Each subsequent dosage to the tank do a 20% wc before treatment.
During summer months would recommend a tank chiller if the daily tank temps vary widely. And unless tank water gets around 55f or less you really dont ever need a heater for these guys.