Avoid using warm water when the temp gets cool. You are much better off with a steady lower temp than a temp that fluctuates. Get a heater once you can. Careful with aquarium salt. Do not over dose. Best to use a smaller half dose of salt. Angels being a SA fish do not like salt. Don't worry about light and feed very sparingly. I'd say about one minute worth of food every other day, split into two feeds. Do 25% water changes on the QT twice a week. This will help beat any ammonia levels.
What were the numbers for the test results on ammonia and nitrite?
Nitrate is the least toxic and actually not really toxic, but rather more of a disease problem when saturated. It may take some time to lower them. Amquel and any other conditioner that states it removes nitrate and even nitrite and aammonia really don't remove these things. Instead they seal the toxicity from the fish. I'm going to assume that there was an ammonia and nitrite spike, and perhaps that is when the angel started to act ill. All ammonia eventually becomes nitrate.
You will want to do several small, but frequent water changes on the main tank. Say about 10% two to three times a week for at least two weeks. Then cut that down to 10% once a week if nitrates read less than 40ppm. Feed the fish about two minute worth of food three times a week split each into two or more feeds. Service the filter. Clean out hoses, impeller housing and other little nooks and crannies waste debris can get trapped. Use tank water to rinse out bio media bags to spare the bio on them. Once the nitrates are below 40ppm, you can up feeds to two minutes daily split into two or more feeds and skip a day or two out of the week. The fish can browse around picking at leftovers. It will help keep nitrates low.
The QT, do 25% water change twice a week to beat ammonia levels.
Well...at least that $40 on an $8 fish will be there when you need it again. Always good to have a QT. In fact, whenever you aquire a new fish, they should be put in a QT for about two weeks before being introduced into the main tank. If the fish is harboring an illness or parasites, it should show up within that two week time period and the fish can be medicated appropriately before introduction. This will help keep disease and parasites out of the main tank.