With any pump picking up stuff coming from a bottom drain would void the warranty except for external pumps with a leaf basket .If you do not need to use the bottom drain all the time why can't you just use a sump pump or dirty water pump plug it in when you need to clean the bottom of the pond and the just unplug it when not needed .I really don't understand bottom drains but was wondering if that would work .I know I have asked people that do pro pond set ups and they don't put bottom drains in because of stone and gravel in the pond on the bottom and sides .Some of them instead will use an air plate under the gravel to keep it clean ,but when I read that was not sure on that either because that may release toxins from under ,but guessing if you keep them going all the time It may not .I get confuse by all the different things to try and some times wonder do they really work .I just use a pool net to clean the bottom .