Good advice from everyone. I agree to change the water gradually to prevent "old tank syndrome", the fish need to get used to good water gradually. Five percent is really safe, and ten might be OK, but be very careful with the vacuuming.
Gently vacuum a portion of the gravel, say 1/3 of the bottom. Keep up the daily water changes, but I wouldn't do a vigorous vacuum each time you do the water change, not yet.
With tons of gunk down in that gravel, too vigorous vacuuming will stir up so much decaying matter that the ammonia will go through the roof, and then the nitrite.
I had this happen to me when I was a noob.
I would do the daily water changes, but would gently vacuum 1/3 of the gravel every couple of days until you've done the whole bottom of the tank, and then after that keep doing 1/3 each time you do a water change.
Prime is an excellent water conditioner because it not only removes chlorine and chloramines, it detoxifies the ammonia and nitrite for appx. twenty-four hours giving you time to do the daily water changes every day.
As you do daily testing, should you find you have detectable levels of ammonia and/or nitrites you can dose the tank with the emergency dose, which is 5 times the regular dose. This takes care of the spike for about twenty-four hours. This gives you breathing room so you don't have to go into desperation mode with an emergency water change. You still have to remove the ammonia and nitrite within twenty-four hours, though.
As recommended you really need to get a liquid test kit, such as API Master. This will help you have accurate readings on the parameters.
A nitrate reading of 200 doesn't mean certain death, but it can cause brown blood disease, which is a result of nitrate converting to nitrite in the body of the fish. Nitrite binds with the hemoglobin in the blood preventing the uptake of oxygen causing the fish to suffocate. A safe reading is around 20 ppm or less. Plants will help with nitrate, once you get things under control by water changes you might add some plants.
Don't add chemicals to adjust the ph. Just allow things to gradually improve with the water changes. Get a glass of water from the tap and let it sit for twenty-four hous, then test it. This will give you an accurate reading of the ph of your water out of the tap. GH and KH reading would be helpful, though the API Master test kit doesn't have the material to test GH and KH, you have to buy that separately.
Feed very lightly so you don't add to the ammonia problems. I wouldn't remove the decor yet. Wait until you have had the filter on the tank for a couple of weeks at least. The beneficial bacteria that exist in the gravel and on the decor will help to seed the new filter media in the filter.
Once you are getting readings of 0 ammonia, 0 nitrites, and nitrates of 20 ppm or so, then you could start to replace some of the decor.
When I bleach rocks I rinse, rinse rinse, and then I let them sit in water with a dechlorinator in it and keep sniffing it until I can't smell bleach anymore. I, personally, would use the regular old bleach with the chlorine smell.... so you can be sure you don't smell that anymore after you've soaked the rocks in dechlorinator.
It's wonderful that you have taken this on, and are going to make the lives of those fish so much better. Good on you for working hard for those little guys.