3 month old silver mollies sick

Question
I have two tanks - 20 gallon w/3 guppies, two silver molly and 1 flame dwark guarami.  The other is a 55 gallon w/1 bala shark, 1 blue guarmi, 1 chinese algea eater, an aquatic frog and 3 female swordtail platies.

In the 20 gallon tank the mollies had a fry of 7.  All were doing wonderful.  Three days ago, I transerred them to the 55 gallon tank because the silver molly surprised me with 13 more fry.

I have lost 3 of the mollies.  They turned a darker color, from white to grey and were hiding in the rocks and under the woods, then next thing I saw was them floating in the filter current.

I am concerned about the others, so I moved them back to the 20 gallon tank.  I also noticed one appears to have a red "bruise" on the tail (not the fin.)

Any thoughts?  Only thing I could assume was they were being attacked by the bala at night?   Are there any diseases this sounds like?

All the other fish appear fine and there readings on both tanks are within range.

Thank you and I hope you can help, I adore all my fish!

Answer
Hi Maryann;

I would suspect the chinese algae eater is bothering them at night. They are vegetarians as babies but very aggressive as they mature and actually become predators to some extent.

Sometimes frogs will nip at fish during the night too but you could be right about the bala. I would just keep mollies out of that tank. They may be "sleeping" in an area that makes them more vulnerable after the lights are out.

It could even be as simple as something in the water chemistry of that particular tank that doesn't suit them. even though water tests "okay", there are trace elements and electrolytes that we can't detect on kits. Mollies may need something that may not be in the proper concentrations in that tank. Hard to say for sure.

At Your Service;
Chris Robbins