If you don't want to go through the drilling process or added expenses of replacing the bottom of a tank, you can go with a simple leca drainage layer set up.

Just place an inch or two of a drainage layer (leca, gravel, pebbles) then a layer or two of fiberglass screen (or fiberglass matting) then your substrate on too of that. Don't forget to add a drainage tube in so you can drain off excess water.