Let us know how your numbers look! CC or aragonite in small amounts will not have a dramatic effect on ph unless you added a lot. Basically, they add KH
(carbonates) to your water which will stabilize ph and prevent it from dropping. It may raise it slightly but this is fine as goldfish do better in hard, alkaline higher ph water.
Any decent lfs
will sell either product in the substrate section (particularly sw
area). They are perfectly safe to use, too! You just add a tiny amount (1-2 spoonsful) in a small mesh bag or a clean piece of pantyhose and stick it in your filter housing. Check the ph in 24hrs and see if there is any change. If theres a big jump, remove a little bit. If it drops, add a bit more. It will be a bit of trial and error to figure out a good amount. Then it simply will need to be replenished every so often as it slowly dissolves. Please ask if you have questions!
PS> Your doing a wonderful thing taking on the care of this guy and I am very sorry about your friend.