Platys slowly dying off in succession

Hello all. So the last of our Platys died last night. Actually 2 died. We started with 7. All females. 1 mom and her 6 babies that we raised. Originally the mom developed a tumor at the same time one of the babies developed one. The baby died within 2 weeks, the mom lived another 2 months. Then almost consistently every 2-3 weeks we lost another of the babies. All the same symptoms. They would all lose their appetite seemingly overnight, they would hide and then die within 2 weeks. Then another would develop the same symptoms and die.
This continued up until last night. The last of the babies had not been eating for about a week. But she was still active. We introduced a new platy to the tank and all
Seemed ok. Yesterday we noticed a white bump protruding from her “butt”. Thought she was constipated so we treated her in an epsom salt bath for the past 3 days. She passed a small amount of poop and thought she was on the mend. However woke up this morning and the last of the babies, AND the new platy were both dead. Anyone experience anything like this with their platies?
Parameters all appear to be fine

 

Hi there, so sorry about your fish

Can I get some more info about your tank?
If you could fill the emergency template out it would be very helpful for us since this seems to be a problem that requires a broader context of your tanks situation.
Aquatic Emergency Template | Freshwater Fish Disease and Health Forum

 

Here we go. See below


Tank
What is the water volume of the tank?
10 gal
How long has the tank been running?
Year and a half
Does it have a filter?
Yes
Does it have a heater?
Yes
What is the water temperature?
79 dees grees
What is the entire stocking of this tank?
Originally 7 female platys, and as they died off we added 3 baby female gollies. Stocking as of the last 2 months would be as follows: 3 female platies, 3 female gollies and 1 Siamese algae eater (non aggressive)
(Please list all fish and inverts.)

Maintenance
How often do you change the water?
1x/wk
How much of the water do you change?
25%
What do you use to treat your water?
Stress coat, melafix, leaf zone
Do you vacuum the substrate or just the water?
Substrate

*Parameters - Very Important
Did you cycle your tank before adding fish?
Yes
What do you use to test the water?
API test kit
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.
Ammonia: 0
Nitrite: 0
Nitrate: 20-40ppm
pH: ~8

Feeding
How often do you feed your fish?
2x/daily
How much do you feed your fish?
1 pinch each feeding
What brand of food do you feed your fish?
Hikari fancy guppy (breakfast)
Hikari tropical micro pellets (dinner)
Frozen peas 1x week
Cucumber 1x week

Do you feed frozen?
Do you feed freeze-dried foods?

Illness & Symptoms
How long have you had this fish?
year and a half from birth
How long ago did you first notice these symptoms?
2-3 weeks before dying
In a few words, can you explain the symptoms?
Lethargy, hiding, not eating, bloated, small red patch on stomach (only noticeable when pulled out of tank)
Have you started any treatment for the illness?
Melafix, epsom salt bath, aquarium salt, 2 rounds general cure
Was your fish physically ill or injured upon purchase?
Raised from birth
How has its behavior and appearance changed, if at all?
Went from extremely active and playful to zombie like almost overnight. Dying within 2 weeks

let me know if you have any other questions that weren’t answered here. Thank you for the help.

 

It could be something contagious. And if I read this all, it's something that starts from the inside. You don't happen to have a photo of those tumors?
First I thought it's bad genetics but the new one died as well. Have you checked your water and fish on parasites yet?

 

we were thinking just bad genetics as well. But yes, the new one dying off put that theory out the window.

Not to sound ignorant but how would we check the water for parasites? So far the gollies that are left don’t appear to have any symptoms neither does the Siamese algae eater. So visually there are no signs of parasite on the fish, but how would we check the water?

One other thing… we had plastic plants in the tank and I always wondered if that may have contaminated the water in any way? We have since removed them in liu of live Anubis

Pictures are attached of the mom and one of her babies who both developed the same type of tumor

The red wagtail is the mom. That was when the tumor was at its biggest. She also developed some slight pine coning just before she passed

then there was her daughter (mixed wagtail and bumble bee) who had the same tumor. She also had a red spot on her side that developed toward the end of her life

 

I think I know what happened. The tumors started at the black spots. A high density of melanophores (dark color cells) can develop melanoma. And most of the time once a tumor starts developing on that spot, it will become bigger. Depending on where on the body they will develop, it can be fatal or not. If it's close to the head, it can push against the small brain. Which will lead to death. When the melanoma develops on the skin close to some other organs, at some point it can push against the organs which "can" be fatal as well. If it's not close to the organs it's mostly not fatal. But it doesn't look attractive either. Depending on how fast it grows the time period before a platy dies, can differ. Once melanoma starts on a dark spot, it can move to other spots that aren't dark. For it moves through skin tissue.

And yes, I do see some the pine coning in pic#4. That would be dropsy and dropsy is contagious. That could explain the death of the new platy.

Testing for parasites or certain bacteria in water, can be done by a microscope.