I have Tiger endlers, about 20 of them, in a 10 gallon tank. Out of my three tanks, that one is running high on nitrates. Unless I do a 40% water change every 4-5 days, it goes to 40 nitrates. Am I overstocked? Is that why my nitrates run high? I only have a few small plants in there so far. Tank is cycled, and has been for years. PH is 7.6, Ammonia 0 Nitrites 0. Thank you.