I am a budding tilapia grower. I am not at all new to keeping fish in closed systems though. The site that you found does have some good info, but steer way clear of hybrids if you want a sustainable system. Notice the owner of the site will sell you a package of two species of Tilapia, but does not sell breeding pairs of either species respectively. Hybrids will not breed true, so you will not be able to retain your own breeding stock, and will have to keep purchasing broodstock from him every couple years. I have a solution, I will sell you 7 blue tilapia for $400. Just kidding. But really, purchase a pure strain. If you lose some stock, you will be able to replace it from your own offspring. I am sure Rex could set you up with some good breeders. Lava rock is decent media, but will become clogged with age. An undergravel filter becomes clogged quickly because waste settles to the bottom. Also, it is underwater, and therefore is not as efficient as trickle filters or wet/dry filters. The bacteria are more efficient in a highly aerobic environment. The exception to this rule is a fluidized bed filter, which seems unbeatable when it comes to eating ammonia, and nitrites. If you are not looking at heavy stocking densities, your design might work. Though for less cost you could fill a 55 gallon plastic barrel up with volcanic rock, drill holes in the bottom, suspend it above your pool, and pump water up above it to a spray bar. This would actually be more efficient than an undergravel filter. If you are looking for heavier densities, you should look into a fluidized bed or maybe the rotating drum wet dry filters. Cecil wrote an article in the most recent Pondboss magazine showing his set up with this type of rotating drum. Don't skimp on biological filtration. Oh also microbes won't just show up to a level that will support fish. You have to either gradually add fish, or preferably feed the bacteria pure ammonia until the system is fully cycled.http://www.aquahobby.com/articles/e_fishless.php This link will describe fishless cycling, it is geared towards aquariums, but you can upsize it. This product will work perfectly as an ammonia source.http://www.aquaticeco.com/customers/subcategories/1716/ProLine-Ammonium-Chloride/ammonia/4 Good luck
