I bought a digital thermometer for my tanks last night and when I took temp readings, both my 10G (tropical, empty and cycling) and my 20G (cold water GF tank) are at 84*F. I know its fine for my tropical tank but, will this hurt my goldfish?
Any filtration should be rated for AT LEAST twice the actual volume. If there is a range go with the bottom end. So the 10 has 5 gallons of filtration and the 20 has about ten gallons worth.
Filters do aerate, but IME bubbles are more effective. They both have air pumps so it is good IMO.
That is the best guide when buying. If you have problems in one of them bump the 20i down to the ten gallon and buy a filter rater for 40-50 gallons for the 20 gallon.
You have enough filtration if: the water is visibly clear (adequate mechanical filtration) and there is no detectable ammonia or nitrite (adequate biological filtration). If those two criteria are met you don't need to buy more filtration yet. However, goldfish in a 20 gallon is pushing it, even with just one. Most likely eventually you may want to increase the filtration.
How many goldfish do you have right now? Any other fish in the 20?
They still need an upgrade. It sounds like it isn't quite cycled yet, so keep up the water changes until it gets there. What type are the other two? Water changes have more to do with stocking than actual tank volume, but you will be challenged to keep a 20 clean enough for two goldfish, especially at that temp.
If you have a small fan just put it over the tank and let it blow the water. But you'll have to add more because this makes it evaporate more. Also filling a water bottle up with water and freezing it. Then drop it in the tank and let it float. It will cool the tank down