Are you using CO2 injection? ... my suspicion is that when you change the water in the tank and increase the amount of bicarbonate/carbonate/CO2 in the water, the plants may just be kicking into high gear photosynthesis... Shawn Keslar, Chemist Nat'l Res. Ctr. for Coal and Energy Analytical Laboratory PO Box 6064 Morgantown WV 26506-6064 304 293 2867 x 5472 FAX 304 293 3749 skeslar at wvu_edu