Thank you for your reply.You can't vacuum nitrifying bacteria away. It is normal for the substrate to have detritus in it, but what kind of substrate are you using? What is the pH and KH? What kind of tetras are we talking about? How much and how often do you feed? When did you add the tetras?
I have small river pebbles. There is an artificial log with live java fern on it, floating hornwort, floating pennywort and a floating plant of watersprite. The plants cover only about 50% of the water surface. I started a fishless cycle April13 th. By Sunday May 12 the nitrites and ammonia were at zero. These remained at zero Monday-Weds. On Thursday May 16 th I added 11 new neon tetras. (One of these had belly bloat and I did not choose him but since the fish were so traumatized by getting caught and acclimating I waited till the next morning to remove him.)
Friday May 17 I did what I considered an aggressive cleaning of any detritus with a python tube. At first I pushed the tube only around the surface but then proceeded to repeatedly shove it all the way to the glass bottom resulting in a fine cloud (like glacial flour if you have ever seen that) flowing through the tube. Lots of it. I did a 50% water change. The next evening I found one fish dead and another too weak to swim being just carried by the water current. Testing showed ammonia 0.15 ppm and nitrites at 0.15 ppm. I did a 50% water change. Then about four hours later I did another water change 0f 20%.
Late Friday (May 17) ammonia was at a trace and nitrites at zero. This morning May 18 ammonia was at 0.25 ppm and nitrites zero, nitrates at 20 ppm. I fear losing all my fish because I know ANY amount of ammonia is deadly. I am not sure how to proceed. There are 8 remaining neons with good color and normal behavior. (One of the neons tend to isolate which worries me but sometimes he seems to consider one end of the aquarium exclusively his and will chase away other neons.)