Ah, that seems the likely cause of the ammonia spike. If you let us know what filter you have, we can tell you how to modify it so you don't need to replace cartridges at all, save you a small fortune, and it'll preserve your beneficial bacteria too, so you're not throwing some away every time you replace a cartridge, risking an ammonia spike each time, especially if overfeeding is happening too.
Okay, so I'd suggest re-testing the water when you get home, there's a very good change ammonia and/or nitrites have risen again since yesterday's water change, so please test again, and if ammonia or nitrites are above zero, do a 75% water change, giving the substrate a good gravel vac at the same time to remove uneaten food and fish poop. Then come back here and answer the follow up questions and let us know the water test results. We can fix this I'm sure
Hopefully in time to save your molly.
Even if you take the sickly molly out of the net to take some clearer photos, it does make sense to put her back in the net afterwards so she's safe from the other fish; healthy fish can and do pick on and chase sickly fish, so you were right to separate her. It's normal for them to do that, sick fish attract predators, so it's in their nature to try to chase one off to keep the whole group safer.
Yet more questions I'm afraid! I'm sorry, but the more info we have, the better we can help you
I'll compile them all together to make it easier for you, can just copy and paste the below questions and include your answers, it'll help! Thank you.
What size is the tank?
How many adult mollies?
Not sure what a tiger algae eater is, is it a tiger pleco? How large is it now?
Accurate tank temp?
GH?
pH?
Ammonia:
Nitrites:
Nitrates:
Filter:
Do you run an airstone, or does the filter agitate the surface enough for adequate oxygen exchange?
How often do you usually water change and what percentage of the total water do you usually remove?
Do you use a syphon to clean the substrate?
If you can include some photos of the whole tank and the other fish, that could be useful too.
I know it's a lot, but since we can't see the fish or tank in person or run tests ourselves, the more complete a picture we can get, the better we can help.