I've had my neon tetras nearly 3 weeks, all was going well till today. Don't know if this is a problem or not, my water has been cloudy a few days but today there is foam bubbles on top of the water! I looked at the filter and there is brown gunk on it(food/waste?). I've been regularly testing the water since I got the fish but still all the readings come up 0ppm, I did a 5 ltr water change 2 days ago! Is this normal, should I clean the filter even though the nitrogen cycle hasn't started yet, or leave it till the tank starts cycling?