Discus Fish are a warm water fish... Temperature is a critical factor in not only keeping discus fish alive but in maintaining a good discus fish growth rate. Optimal temperature for Discus Fish should be between 84-90 deg. Temperatures Below 82 should be avoided. We keep all of non-breeding discus fish at 88, and breeders at 84... The hatch rate of discus fish eggs greatly diminishes above this temp. The growth rate in younger discus fish raised at 88-90 as opposed to 82-84 is unmistakable. Along with the fact that many pathogenic parasites cannot survive above the 86 threshold in a discus fish aquarium. It should never be a problem for the discus fish owner to use 1 cup of non-iodized salt (I prefer Morton blue bag water softener salt... 99.5% sodium, no water softner conditoners) per 100 gallons of water and kicking the temp to 90. This will knock the ick into oblivion.