Too much circle of life my friend...I have 2 guppies and 2 ottos in a 40L tank and I never feed them...seems a bit too much especially when I do 70% water changes twice a week and I siphon alot of the dirt.....
Hi Jaap
Dirt comes obviously from fish, plants melting, substrate and everything you have or you put into your tank... I have seen you struggling with all sort of algae problems and now that I can see these pictures I bet this is your problem. It was yesterday when I explained in another thread that I am dismantling my 60 cm tank after 2 years and 2 re-scapes... and man, you won't believe how much dirt there was at the substrate level! I also do 2x50% weekly WCs, violent plant shaking, siphoning, etc. but the truth is that you cannot reach all the parts in your tank, the rhythm of debris production is higher than your cleaning, we are not efficient enough doing that job... or any combination of them. And this increases the chance of having bring all sort of algae... especially BBA if you are on the high tech approach. One year ago I could leave home for 10 days and come back with the tank looking good. Now I definitely need the 2xWC weekly, thinning, H2O2 spot treatments to keep it in good shape (=no visible traces of algae, BBA)
Another interesting thing. In my nano tank (barely 20 liters) I only have cherry shrimps, around 20 of them... Plants are in perfect shape, no melting, not a single death. In 40 days I have clean my filter twice and it was plenty of shrimp heces and even a kind of gunk (shrimp heces paste?). If 20 cherries can do this, your guppies are like a cow in your bath. Your plant problems do contribute for sure to this situation.
I think I suggested this many posts ago... but my feeling is that rebooting a tank is sometimes much easier. Even if you reach the sweet point with your plants now, dealing with all this stuff and algae bits and spores will make things more difficult.
Jordi