Hi! I think this qualifies as a getting started question, so here goes.
I have a 20 gallon planted freshwater tank that has been up and running for 2 1/2 years with 3 pink zebra danios, 2 otos, 2 female sunset platys and a few fry. The platys are the root of my problem. When I got livebearers, I guess I didn't realize just how many babies they could really have! So after many trips to the LFS
with bags of platys, I have rehomed all but two of our big mamas. I guess I have a great bacterial filter in the tank because after being severely over populated for a long time, when I tested water params after no water change, it was Ammonia 0, Nitrite 0, NitrAte 40ppm. Even with the overcrowding I have had little disease, and few deaths. I have had a few cases of what looked like chronic wasting, something that looked like ich, but only affected 1 fish at a time, and lost 2 danios to what looked like TB.
I just recently rescued 2 goldfish, 1 common, 1 comet, and currently have them in my spare 2 1/2 gallon, but they are growing rapidly. I traded my platys for a 10 gallon and would now like to move the tropicals into the 10 gallon, and the goldfish into the 20 for now. The 10 gallon is cycling nicely, the goldfish are beautiful, healthy and sweet, and all the fish in the 20 are healthy and happy and get along great.
So here's the question. I want to move the goldfish into the tank that is well cycled and established, but there has been some disease in there. Since it is planted with healthy, well established plants, emptying and sterilizing the tank is not an option. How can I ensure that my goldfish will not get sick when I move them in?
Thank you for any advice you can give!