Actually, salinities kept at levels of 1.018 don't deter any diseases--it may actually promote more resistant strains of diseases, making them more difficult to treat via hyposalinity, necessitating the use of copper and the like. The only thing a level of 1.018 would do is *very slightly* reduce the amount of energy expended in osmoregulation. It is only when you approach levels of 1.010-1.009 that you start seeing disease prevention/treatment and serious declines in osmoregulatory energy expenditure.
Temperatures are a touchy subject--each species seems to have a "preference." Many clownfish species are usually found in 80ish degree water, due to the incidence of anemones. Hawaiian yellow tangs, for example, were found to prefer temps of around 74 degrees, but only at night. That is why I gave you that range, because anything within that is fine. For a simple fish only, I'd shoot for 77-78 degrees. You have higher oxygen levels with slightly cooler temps, and with a higher fish load, that helps.