So many of those fish will outgrow your tank. A single angel in a 20gal is too much.. A single goldfish in a 20 gal is too much as well.
You don't have to buy products to change your tap water, in fact, it's best for the fish to leave out all the additives (except obviously a conditioner to remove chlorine). You should test your tap water and buy fish appropriate for the water. My fish are all south american fish that require soft, acidic water, because that it was naturally occurs in my tank.
You are keeping fish together with different care requirements. Livebearers and goldfish I believe as well need hard water, they suffer without those minerals in the water. Fish that need softer water can, for instance, die of calcium blockages that form because they are not adapted to those high mineral contents.
Plus, like your cory fish should be in a larger group than two.. If those are a sort of pygmy cory you should really have at least 5-8.. There's no room for more than one pleco with so many other fish, those things are poop machines!
I'm not trying to be rude, you are on a forum about fish care.. We're replying to try to help you provide adequate care for the fish who's lives are now in your hands.
You may want to go through and read the profiles for each fish you have stocked so you can see each individual fish's requirements and see if that matches what naturally occurs in your tank. And if your temps get up into the 80s (which is horrible for goldfish by the way, they are cold water fish) then pick fish appropriate to that. Rams will do fine in a 20gal with 80 degree temps, there are a few cory species that can as well, cardinal tetras are also an option as well as others. Pick fish that match what you can easily provide so you have less to worry about and less work to bother with.
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Last edited by jennesque; 06-11-2013 at 12:44 PM.