Loosing a fish almost weekly

For starters, I'm not new to fish.... I have a 55g tank, water tests fine, I have ruled that out already.
Tank has guppies/platys in it... Recently I added 2 super pregnant molly/mosquito fish hybrids to it (I breed them as feeders)... Being that they are mostly colorless, I'm wondering if what I'm seeing on one of them is what's killing my other fish... The back half is a pinkish color -right one in pic-(bacteremia/septicemia??)....
Fish that die act listless for a day or 2 then just die, I've started removing any that don't act normal as soon as I notice... Thoughts??

Pic since it didn't post w post

Pic since it didn't post w post

Any picture?
Can you described any other symptoms before they died?

What are you testing for can i ask and with what brand of tests?

Im not the best at reading the colours and found some brands just aren’t accurate.

Make sure you cover the basics as well -
is the filter blocked?
is there a dead fish or something jammed in the intake?
are you testing for the things below?

Fish back right, pink back half...

Pic below...
Only symptom is about 2 days of "sulking" (not really eating, staying in 1 spot)

I have two test kits one dipstick and one chemical I don't remember what brand off hand I'm at home... I also have the pet store tested just out of curiosity, it always tests fine.
Filter is clean there's nothing in it. Half the rock for sand about 2 months ago see if that would help, 50% water change.... It's been going on for like 6mo to a year maybe...
I've tried Mila fix and FEMA fix and some sort of parasite treatment (don't remember what I think I still have the box at home)....
Live-bearers so endless supply of fish but goal is to breed them to feed and I don't want to feed if they're going to transfer something to my eels

The fish in the picture looks like it has a bacterial infection on the rear half of the body (pink area). I would add salt, (see directions below).

You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 2 heaped tablespoon per 20 litres of water.

If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 4 heaped tablespoons per 20 litres.

Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.

The salt will not affect the beneficial filter bacteria but the higher dose rate (4 heaped tablespoons per 20 litres) will affect some plants and some snails. The lower dose rate will not affect plants, shrimp or snails.

After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.

If you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.