I have a 15 gallon tank with two medium sized sponge filters 5 sparkling gourami and 11 dwarf corydoras. It's been running for about 6 months but yesterday I did a check and my ammonia is at 0.5-1ppm. I feed a small pinch of flake+3 corydoras pellets once every two days, and have a heavily planted tank. The only thing I've changed recently is cleaning one of the sponge filters, I emptied some water from the tank into a bucket, took the sponge out and rinsed it in the tank water, put more water from the tank in the bucket, repeat. I tested my nitrates today and they were zero, but I also have lots of duckweed so suspect that might be causing that? I've added seachem prime but in the meantime, any advice?