No new livestock. I've had both fish since October. I replaced the filter 2 weeks ago. Saturday I did amount a 15 percent water change but I used seachem prime like I always have. I'm puzzled and saddened that it appears my Molly will probably die
When you say you replaced the filter, did you mean you replaced the filter cartridge?
No cats and no aerosol. It was like 3
Yes just the cartridge
I'd put money on a bet that that is why you have an ammonia spike. The nitrifying microbes live on surfaces where the highest levels of oxygenated water is and that most often is in the filter. The filter cartridges is where the majority of the surface where they will be. When you replaced the old cartridge, you threw away a big colony of your biological filter bed. In the future, even tho the companies say to replace the cartridges, do not replace them until they are literally falling apart. Gently rinse them off in tank water to get the "gunk" off without destroying the microbes and then put it back into the filter. This is best done when you do your water changes so that you have tank water you are getting rid of. As long as you have water going though the cartridges, you are good to go.
When you see the cartridge is starting to fall apart, place a replacement cartridge between the old cartridge and the outflow so that the nitrifying microbes can transfer from the old cartridge to the new one. Do this 2-3 weeks before you totally replace the old cartridge. The longer the better so that your new cartridge gets a good colony growing in it. That should prevent any ammonia spikes as long as you don't overload the tank.
What matters is the amount of oxygenated water is in the area, not how much bacteria holding materials are present. For example, if you were take a simple 10 gallon tank and placed a filter cartridge inside the water on one side of it and a small air stone on the opposite side of the tank, the majority of the nitrifying microbes would be near the air stone, not the filter cartridge. There is a very simple way of replacing those HOB filter cartridges without damaging the nitrifying bed and that is to put the replacement pad in the filter for some time before removing the old filter. And with that said, the old method of depending on carbon or charcoal for cleaning the water has kind of gone by the wayside and the push by HOB companies for changing those cartridges seems more a means to sell more product than it is for the health of the aquarium. This proved out with the "invention" of the DLS material first used in saltwater tanks many years ago. They had zero carbon or charcoal in the setup and the tanks remained healthy without them. You never replaced that material until the floss part was literally falling apart which took months to years. The microbes would be sitting on the outer "screen" that DLS used and would transfer/ repopulate onto the floss with time as oxygenated water went through it. So the 2 is better than one idea you are expressing is not necessarily the best. It all depends on what you are using. And with that said,
in my hatcheries, I used both a sponge filter and a box filter in each tank. This was not for extra filtration but for security if one of the filters failed to work ( i.e. sponge filter got clogged, box filter airline got clogged ), the other was running. The only way they both failed was if the compressor went down.
