well, i guess the verdict is in and there seems to be unanimous agreement. i am still not 100 % convinced though; it's just not enough for me to know that salts, bacteria levels are raising and this can be harmful. i would like to know for myself how much is harmful, and so i will conduct a side by side experiment, changing the rez in one tub every 2 weeks, and topping off with nutes in another tub without ever changing. i'll let you all know what my findings are. thanks for the tips. ciao