Jan, I don't have any prime at the moment so can't test this for you. But from memory Prime did cause a low ammonia reading when I used to use it. It doesn't actually contain ammonia so you should be fine.
Now that you know everything is clean I would just keep to the weekly water changes and make sure I am not overfeeding. Use 0.5 as your "baseline" ammonia reading and only take action if your ammonia goes up from this. From recent posts in this thread the fish seem healthy and behaving normally.
If you have seen the recent thread on the dwarf chain loach I had to re-home on the weekend I am following this exact advice from @Byron. In summary last night I had an unexplained 0.25 reading. My other tanks have 0 and so does my source water. So last night I fed my fish (only a little) and tested again this morning and the reading is still 0.25. So I am going to keep monitoring and do nothing else. With 6 fish in a 20G, and the fact that I fed them last night the ammonia would definitely have gone up if the filter was not dealing with it. I am in exactly the same boat as you as the tank itself is sterile as the tank and substrate are brand new and have no bacteria on the surfaces. So I am relying on the sponge filter that I moved from my community tank and the fact that I have a lot of floating plants. Because it was an emergency move I have no plants in the substrate yet apart from one tiny amazon sword.
You don't want to strip down and scrub the whole tank regularly as a lot of beneficial bacteria will eventually form on the glass and substrate.
Now that you know everything is clean I would just keep to the weekly water changes and make sure I am not overfeeding. Use 0.5 as your "baseline" ammonia reading and only take action if your ammonia goes up from this. From recent posts in this thread the fish seem healthy and behaving normally.
If you have seen the recent thread on the dwarf chain loach I had to re-home on the weekend I am following this exact advice from @Byron. In summary last night I had an unexplained 0.25 reading. My other tanks have 0 and so does my source water. So last night I fed my fish (only a little) and tested again this morning and the reading is still 0.25. So I am going to keep monitoring and do nothing else. With 6 fish in a 20G, and the fact that I fed them last night the ammonia would definitely have gone up if the filter was not dealing with it. I am in exactly the same boat as you as the tank itself is sterile as the tank and substrate are brand new and have no bacteria on the surfaces. So I am relying on the sponge filter that I moved from my community tank and the fact that I have a lot of floating plants. Because it was an emergency move I have no plants in the substrate yet apart from one tiny amazon sword.
You don't want to strip down and scrub the whole tank regularly as a lot of beneficial bacteria will eventually form on the glass and substrate.
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