Mostly for the plants, I find that they grow easier. Will my plants still grow with such low temperature?This is much too warm a temperature for neon tetra (which you subsequently said you acquired). They should not be maintained above the mid-70's F because they will weaken and succumb to this and that. Cardinals would be fine at such warmth, but not neons. What fish are in this tank that need such a high temperature?
Temperature is very important for fish because they are ectotherms and the temperature of their environment waters drives the metabolism and the acceptable range for most species is quite limited.