Okay I counted twenty fish in there. If you put them in a way bigger tank it may help. Twenty fish need 200 gallons ever more for comets which will make that 300 gallon. Get a giant 300 gallon tank!!!!!

I agree, way too many fish in there.  Fancy goldfish(the ones with double tails) need 20 gallons for the first fish and 10 gallons for each fish after that, they get around 8" on average.  The common and comets need 20 gallons PER fish, they get around 10-12".  You need to just have a couple fish in that tank.   These fish look small now but grow fast and BIG.  We don't recommend mixing the single tails with the fancy ones because they are so much faster and the fancy guys will have a hard time competing for food.   Try to re-home or return some of them and that will greatly improve their chances at survival.  Keep 2-3 only. 

Also, your tank is not cycled yet and they are probably suffering from ammonia poisoning right now.  You need to do LARGE water changes,  take out like 80% of the water and replace with fresh water, make sure to add declorinater/water conditioner first before adding the water and try to match the temperature of the water as best you can.  I use Prime as a water conditioner and it works great, it will also neutralize the ammonia/nitrite.  Add it based on the directions for the whole tank.  You need to get some test kits for the following:  ammonia, nitrite, nitrATe, and ph.  ammonia needs to stay below .25 if at all possible, if it goes above that then do a large water change adding the Prime again.  The nitrite is the same way.  Read up on how to cycle a tank and that will help u understand a bit more of what I am saying.  The nitrate will be the last thing that you will get a reading on and may take up to 4 or 6 weeks before you start seeing it.  This is the not as toxic bacteria but you still want it to stay in the safe range,  I try not to let mine get above 20.   When you get that showing results then your ammonia and nitrite should be showing 0, then you are cycled.  If you can keep the levels low enough the fish should improve.  You need an air stone in there also, get a pump from you local pet store or wally world or somewhere and a bubble wand or air stones.  They REALLY need the oxygen.  Don't medicate right now as that can sometimes cause more harm than good.  Fix the water problems and that will fix most of the fishes problems.  If the ich continues then you can use aquarium salt to help with that, look up how to use it online if needed but it should say it on the box, just make sure its fish safe aquarium salt can get it same places I mentioned before.