The carbon filter needs to be changed every two weeks, but don't change out the rest of the filter media until it's actually very sludgy. It's only better for the system if you have the beneficial bacteria intact!

I'd agree with scrivens though, maybe you should check if your fish-sitter could change some water while you're gone?

I'm a little concerned about the water smelling "horrible", it should never smell horrible! In my cases, the tanks only smelled bad if I had a sick fish with strong meds in it, or in case a lone fish died and had the water polluted over-night. Something would definitely have to be wrong with it to start smelling so bad! But I bet that later the easy balance and stress zyme will help tremendously

It definitely would be, so when you come back, change out a little water in the small tank and keep an eye on the water parameters of the bigger one, once the ammonia and most of the nitrite is gone, feel free to add the fish But there might be a small nitrogen spike when you add the goldfish, so you'll need to carefully monitor the water quality (read: test it every day for the first week or two) and change out 50% whenever you notice any ammonia in the tank! And then you'll also want to add beneficial bacteria with each change

o.k. our plan is to let the new tank cycle and then add the fish. That should be sometime in January.

He's doing lots better today. He has a wiggle dance that he does when he wants you to feed him and as soon as he saw us this morning he started doing it again. 

Thanks for all the help and good advice! Have a wonderful Christmas!

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A update on a new thread with some more questions!