I am not 100% sure of how your bottom drain pipe is situated in your tank, but probably the best idea I have seen was where a person came into the bottom of the tank, then added a 90 degree elbow and ran a pipe horizontally about an inch or so of the bottom of the tank, that went as far as he could without hitting the side of the tank. He then had placed a cap on the end of the pipe. But how did the water get into the pipe you ask?? He had drilled holes along the bottom side of the horizontal pipe, facing down towards the bottom of the tank and his fish were to large to go under the pipe, so it was impossible for them to get sucked up, but the water drained well. Also with multiple holes, even if some got clogged by something there were plenty others to keep draining. You could always do this and even place a mesh box over the horizontal pipe if you have really little fish, to keep them away.