I recently bought a foreclosed house that has an about 700 gallon pond built into the deck. It was sitting for some months unattended, but the realtor had been feeding the fish.
The water was horrible, you couldn't see into it. No filter, no oxygenation. There were some lilly pads, which I suppose helped with oxygen during the day.
Anyway, inside this pond was 7 goldfish (comets, I think.. 1 inch long at the most.) 1 lion head goldfish, and 3 decently ugly koi 5-6 inches. How they survived several months like this, I have no idea.
I emptied the pond, cleaned all the muck off the bottom. Left most of the algae on the liner. Refilled the pond. I put in a filter with a fountain attachment. I also bought a solar powered oxygenator which runs from about 9am to 6pm. I put in some barley extract. I can see to the bottom of the pond now, the water is decently clear.
Is there anything else I should do? As much as i'd prefer nicer koi, the personalities of those in the pond have grown on me, so I'll probably just live with them. I just want to make sure I can keep them all healthy.