Personally, I would never tell anyone it is OK to put anything in a fish pond that will trap debris that will decay. Think about it. When was the last time you saw Kohler put gravel in the bottom of a toilet?
Follow those toilet vendor's lead. Smooth walls and floor. Rounded intersections between walls and floor (no right angles that would let stuff settle out and making cleaning more difficult). Hole in the bottom to flush out the waste. There is NOTHING positive about organic waste decaying in a pond.
Don't believe me.....don't flush your toilets.
You will not like...and neither do the fish. Clean water is healthier for both of you.