Update:
I'm loving the 75 gallon tank, and things have been great up until about 2 weeks ago. This new thing that started is driving me crazy...my water is dirty!! I've never had an issue with dirty water since I've kept fish. It's not cloudy...there are actually billions of visible particles floating in my water and it looks awful! I think I've been making the rookie mistake of overfeeding. I'm not used to goldies and I've read that they should be fed twice a day. They get pellets in the morning and either gel food or frozen something in the evenings. Even weirder though, I swear the other day that mosquito larvae was floating around. The fish went nuts and ate whatever they could find. When I cleaned my filters, there were tons of tiny worm like things on the sponges. I don't know what in the world is happening. I have an inch to two inch sand substrate, and I know that goldfish are sifters, so could it be that they are simply stirring up debris on the surface of the sand? I cleaned all my filters except one. I don't know how to clear this up. I considered putting the fish in a temporary tank and starving out the planaria or whatever those worms are. As of now I've cut back on feeding dramatically. Any one else experience this? The fish seem perfectly fine.