I have a small preformed pond. It's the one that came from Wal-Mart. I think it holds 80 gallons. Anyway, I installed it last year, didn't do much else with it then. This year I have emptied it, cleaned it out and refilled it with water. I put some chemicals in it to treat the tap water, then I put in two gold fish. My first question is what do I need to know to keep this thing clean and going? It has been up and going for three weeks. The fish seem fine, but the water is getting dirty and algy is building up everywhere. I would appreciate any chemical or fish advice that you could provide. Thanks, p.s. I really would like to add some little sharks. Wal-Mart sells some that grow to 4 inches or so. It says that they prefer salt water but don't have to have it. Has anyone tried to raise them without the salt water?