We have a recently established freshwater tank (30 gal) with biowheel filter, aerator, etc. We cycled the tank for about a week before adding fish, and it used to house goldfish with its previous owner and was never quite dried out before we replaced the water and cycled it to start our tank.
We don't have anything overly complicated in there (two mollies, a few neons, tiger barbs, and a couple of zebra danios). The problem is that one zebra danio has began to turn yellow where they all used to be white. I've done a little googling of danios and apparently it's normal for them to have yellowish coloring--but I am worried because the rest of them don't seem to be having the same issue. Also, we have a white molly with big fins, and the tips of it's fins seem to be yellowing as well. I am wondering if this is something i need to worry about and if so how to rectify the situation. Thanks for any help you can give.