Hi
goldfish! Welcome to FishLore
I'm really sorry about the loss of your fish.
I'm going to give you a link to read about the
Nitrogen cycle
It's very important for you to understand what's going on in your tank right now.
Cycling can take up to a month. During this time ammonia (from fish waste and left over food) and nitrites can reach toxic levels.
While the fish is trying to fight the outside stressors (toxic water) it weakens their immune system and leaves them open to illness such as
ICH and possible death.
It's quite possible they were sick when you got them. They're not kept in the greatest conditions which also weakens them.
In order to try and get your fish through the cycling process, it would be a good idea to begin daily water changes. Use Prime as your water conditioner. That'll detox the ammonia for 24 hours between water changes.
Continue to do this until your tank is cycled. Your readings will be 0 ammonia, 0
nitrite with some nitrates showing.
Be patient, it'll take some time.
Now, for the ich, you can slowly raise the temp to 84F. Leave it there for at least 2 weeks.
Increase aeration to make up for less oxygen in warmer water.
Do thorough gravel vacs while doing your daily water changes to pick up any ich spores in the gravel.
It might be a good idea to sterilize the gravel vac between uses to kill off any spores.
All that being said, goldies are huge waste producers and can grow really big. 29g isn't big enough for 4 goldfish. It's recommended 20g for the first goldy then 10g's for each additional goldfish.
Best of luck, I hope they feel better soon.