Hi Marisa,
I recall going thru a lot of goldfish as a kid & it was always a royal pain to keep them alive. They literally have no immune system so any little changes to the water (temp, Nitrates, or otherwise) can make them go belly up. One thing that was suggested when I was keeping them was to boil anything you want to put in the tank before it goes in the water, whether it be gravel or fake plants. Don't put real plants in the tank initially because those can also carry disease (I lost a lot of fish this way). This doesn't mean that you can never put real plants in there, just initially until you get the hang of it. And wash your hands thoroughly before reaching into the water to do anything. Sterility works wonders.
If the water changes temperature a lot (as it does during Fall & Spring), keeping the temp constant will help this. If I remember correctly, goldfish like a cooler water temp than RES's....around 70....I'm going off memory here, so definitely confirm this exact temp.
Keep an eye out for fin rot ("ich") daily and keep some fin rot medication on-hand at all times. This is a very common ailment among goldfish. Catching this at an early stage will help you solve the problem very quickly.
Some electrolyte water conditioner (blue liquid, clear plastic bottle....can't remember the name but I have seen it everywhere) is what I use for my Betta & frog & that helps get rid of chlorine & other nasties. Every time the water is changed, I add 1-2 drops per cup of water & so far my aquatic babies have done well.
What kind of goldfish are you going to get? Are you going to get some of those fancy Chinese kind with the bubble faces? Those are so cute!
