Is the 15L cycled? And what else is in there? I'd imagine you'd have to do frequent water changes on it to keep the ammonia levels down. It is going to take a while for you to get the tank ammonia down to 0. What I was saying is if the 15L wasn't cycled do large water changes and add an ammonia remover. I'd wait for a second brain to confirm this process.
Very likely ammonia poisoning. It can kill a fish very quickly. Only thing I can think of doing is move the filter(or spare filter?) to a bucket or spare tank or something to let it cycle(adding a few drops of pure ammonia every few days) and doing large water changes on the tank that your fish are currently in daily , this removes ammonia and could prevent the fish from getting more sick. Adding something that does remove ammonia into their current tank could assist. This will take a long time. 6 weeks possible. Read on how to do a fish less cycle on google may help.
Good luck
Matt
Yeah I saw one at this ladies house, she breeds fighters, and she had a huge(8cm+) gold apple snail. It was at the top of the tank and dropped off, was seriously cool! She had a massive ramshorn aswell...
Huh that's really weird. You hear about acrylic tanks being made all the time, but there's nothing aquarium suitable? I'd love to hear how it goes.
Good luck
Matt
Since you have fish in there, you don't want hpthem to get ammonia poisoning. But you want some ammonia in there for the bacteria to grow. I'd say 20% every 2-3 days. Maybe a google search will help. You don't want to be adding anything that removes ammonia either wise the bacteria won't grow. Just something to remove the cloramines and chlorine.
Ahh there's your reason. You've most likely killed all your BB(beneficial bacteria) wen you turn your filter off. Therefore recycling the tank starting at ammonia.
It doesn't really matter. Water at a certain temperature can only hold so much oxygen. Providing that your tank is getting enough surface disruption it should have enough oxygen. Adding plant does not always help. During daytime they do photosynthesis turning CO2 in to oxygen. At night it reverses oxygen into CO2 . I think it may be ammonia. Do a water change. Also when you bought your WCMM were they cold or tropical? Did you acclimate them?
Good luck
Matt