I would make one of those giant clay caves/mountain. They are DIY, and look great, but you need to find fish safe glaze to color. I had seen a link on how to make it a year ago, but I can't find it. But your fish would love it, if you could make one. First, you need non-tox clay, and then you need a lot of it to make something with a base of 32"X32 with a height of ~4ft. Then, you need to make twisting branch like loops with space for fish to swim through, arches, caves, etc.