Fishy...
So many questions I'll have to go back and forth to answer them. But 1st would like to say that you do need as much information as humanly possible for us to help you. One missed variant to the problem could change alot of possible causes. I'm going out on a limb but I think you have high nitrites which cause the fish to not be able to get enough O2. This will cause red gills and gasping at the surface. High ammonia will also but nitrites take longer to establish so I'm going to throw that in 1st as the reason.
Lets tackle the waterchange/water temp 1st.
You don't really need to keep the heater on. When you do a water change just try to get the new water to the same temp as the tank water before you put it in. It takes some practice but I can go to my faucet and get the handles to the exact position now for each tank.
You don't need to take the entire hood off to do water changes. Just lift the front and stick the gravel vac in, do half the tank one week and the other half the next. All fish get skittish at 1st but learn that they aren't in any danger and actually start swimming around the vac. Water changes are they most important part of fish keeping. Without them all your parameters(pH,Am,Ni,NA) are in danger of becoming toxic to the fish and killing them.
Test Kit
Go to Petsmart.com go to fish then test kits and find the AP Freshwater Master Kit, print out the ad and take it to the Petsmart in your area. They will match the online price and you'll get the kit for under $15.
This really is the 2nd most important part of fish keeping. If you don't know what the water is doing you can't help your fish.
Lights on
If you keep turning the lights on and off that will be more stressful to them. Turn the lights on and leave them on all day.Just like any thing else they need to get use to the lights. Its not stressing the fish as much as you think. Give them some more hiding places, leave the lights on, and you'll find after a few hours they will be swimming all over the place. There will be times you think they are hiding, my fish usually chill under the plants around 12-2 and then become active again. I call it hiding from the midday heat.
Diseases
Google fish tank diseases and you'll find some sites that will show pics. A lot of diseases can be stopped or prevented simply by good water quality.
If you want further accurate help you'll need to give us more information.
Tank size,type of fish in the tank,decor,filter and cleaning ritual, water change (how much,how often), water temp.,and of course pH,Am,Ni,Na.