Ditto on using the existing pond water. I had a kiddie pool, the dog pool, and emptied a bunch of storage totes into the spare room, and collected rain water (we got like 17" in a month!) to help avoid using tap water. The fish did fantastic!

Here is how I built mine. I used 2x6's. The corners are these, and they are PERFECT. My pond is only 18" above ground, the rest is dug in. (I wanted the soil heat for winter.)