My aquarium has long since cycled and has not required water changes (even for nitrate) as Quick Dip and Freshwater Master Test Kit give constant readings such as "0" Ammonia, "0" Nitrite, "20" Nitrate, "75" Hardness, "120" Alkalinity, "6.8" Ph @ 76 degrees temp. I do weekly add enough rainwater to replace water lost via evaporation. Advice I have garnered indicates Nitrate should be somewhere around 4ppm. So, I desire to DIY a 'flat" denitrator using some type of corrugated plastic such as Lexon or perhaps coroplast. I have the inclineation, the time but the wherewithall is my problem. Would some type of acrylic double skinned sheeting be worth considering? Any members input would be appreciated.