Oh, now i understand the cat wall... sorry, abit slow at times... 3" seems abit low, mayb 5-8"? Note, not only cats, but if ur keeping fast swimmers, they may jump out.

Since ur in planning stage, may i suggest an idea? how about raising the pond above ground level and place a side glass to the wall... then u can view ur gfs from side and top view. Sweet!

From my observation, if u like watching ur gfs, then u may want to consider how u plan to watch them... standing up? or sitting down? which in-turn decides if the pond should be below or above ground level... since gfs r not large, i tend to watch them quite closely, like near the waters edge... Unlike watching big kois in a swimming pool size pond, standing up is no prob... but i can just imagine standing up watching gfs in a below ground pond... hmm, they look pretty tiny from way up here... am sure they would look much better if im sitting on the floor.

Apologies if im rambling too much n not making any sense

Happy planning