I have had my 20 gallon tall for over a year now but stopped testing after 3 months. I recently tested and ammonia and nitrites are at 0 ppm. I forgot to shake well for nitrates but I retested and got 30-40 ppm. (Using a liquid test kit). I think this is also the problem with my algae that just won't budge. Normally I w/c once a week (on Sunday's) but changing this to 2 times a week (sundays and Wednesdays) should help bring this number down to something a little more comfortable, maybe 20 ppm. Thanks in advance for replies.