I used some test strips today.
Ammonia and nitrite were at zero, which was expected since I use biological filters and my tank cycled a long time ago.
Nitrates were around 30 ppm. The strips are the kind where they change color and the color was between the 20ppm and the 40ppm colors. That was after a water change. This means that I have to change the way I’ve been doing my tank maintenance.
I always do tank maintenance on sundays, not counting the times were siphon clean any waste sitting at the tank bottom. On the first sunday of the month, I do a 100% water change and scrub everything down. I change one or two gallons of water the remaining sundays after that. Every sunday, I clean the sponge filters under running water and clean the biological filters (ceramic rings and lava rock) with old aquarium water.
I did some research and it seems that 40ppm of nitrates should be the max in turtle tanks. I should do an 85% water change when nitrates reach 40ppm, which will reduce it to 6ppm. At least according to the turtletimes forums.
I reached the 30 ppm level today after a week, and that was with a one gallon water change. I have four gallons of water (too little, I know – a bigger tank is coming soon!!). Since I changed a gallon, it means I actually had 40ppm before the water change!
From now on, I have to change all the water every week!!