Ive seen these things in many mags/catalogs/www-sites/etc. But they are expensive, too much so for me. BUT I would like to be able to benefit from them also(without paying the 100+ usd to get one). I noticed that the replacement parts are just pennies, compared to the total cost. A filter sleave and the filter material for one was about 10-12 usd... now the rest of the system is something to draw water into the fitler/through tthe material in the sleeve/ and then back out into the tank... NOW>>> I think you could make one out of a cheep small pump and some pvc.. but are there any people that have done this???