Have you only got the AquaClear filter on the tank?
How often and how do you clean the filter?
What sort of filter materials/ media do you have in the filter?
What did you feed the fish yesterday and today?
Have they had anything different in their diet?
If you use frozen food, is it frozen when you get it home, and is your freezer working properly?
How old is the dry food you feed them?
Does it look ok and feel dry?
Dry food can absorb moisture form the air and go mouldy and this can kill fish. Try to use dry food within one month of opening it and if possible, within 2 weeks of opening it. If you have a lot of dry food, freeze most of it and only keep a small amount out for the fish.
Has anything new been added to the aquarium in the last few weeks?
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I doubt this is Columnaris. None of the fish have symptoms except the Melanotaenia lacustris rainbowfish and that looks more like a physical wound or an external protozoan infection like Costia, which won't respond to antibiotics.
The white on the tip of the dorsal fin of the lacustris rainbow is excess mucous and the damaged area also has excess mucous on it.
I would not be adding medications at this stage because diseases like Columnaris and Costia don't wipe out a bunch of healthy fish in one day and leave no external symptoms except on one fish. And both of these diseases occur in dirty tanks and this tank gets a 50% water change every 14 days (last water changes was 10 days ago), which should be sufficient to keep it clean.
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Regarding the stringy white poop. I wouldn't worry too much about that yet. The most important thing is to stop the fish dying and then you can deal with this after, assuming they live.
Stringy white poop can be caused by an internal bacterial or protozoan infection, or intestinal worms. There is more information about the causes and treatments in the link below.
Fish do a stringy white poop for several reasons. 1) Internal Bacterial Infections causes the fish to stop eating, swell up like a balloon, breath heavily at the surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be...
www.fishforums.net
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If your aquarium doesn't have a cover on it, then the cat might have contaminated the water.
Has the cat been treated with anything (external flea or worm treatment, medicated shampoo, etc) in the last few weeks?
Has the house or property been sprayed for anything in the last 12 months?
A lot of pesticides are lethal to aquatic organisms and last for years. If the cat has come in contact with these chemicals, and put its paw or tail in the water, it might be enough to wipe out the fish.
At this stage I would be trying big daily water changes and try to clean the sand with a gravel cleaner. Maybe add some carbon to the filter and then hope for the best.