Stand for goldfish

Here's my suggestion... these illustrations aren't exactly to scale.

Start with simple three-wire stand, possibly painted black. Then you can paint a standard 12x12 concrete paving slab to match your pond bottom, and you can just set it down on the base of the stand. No nuts and bolts required, easy to move when necessary, and it should be very stable.

The angle of the fish can be determined by the length of the wires, when I make the stands.