Illnes and now eggs within my fish tank

Question
I was hoping you could help me. I have a 22 litre tank which has been stable for 3 months with regular changes. It contains one guppy, 2 gold fish and 4 danios.
Last week i purchased 2 new plants and 2 baby red platys from my local fish shop.

The next day my other fish started to become ill. The would stay near the bottom or gasp at the top. One danio and one goldfish lost swim control and died soon after.

I revisted my fish shop where i was advised to buy an anti bacterial to add to the water (they also ran tests on my tank water and said it was okay. I carried out a 25% tank change anyway as they recommended.

The next day 2 danios and the final goldfish died over night.

The tank now only contains the guppy, one danio and the 2 new fish - which have lasted 2 days with no more illness so far.

This morning i looked in the tank and it appears eggs, or white sacs are now growing on some of the pieces of gravel.

Can you please help me?

Kind Regards
Lee Anne

Answer
Hi Lee Anne,
If the fish has trouble maintaining their balance they probably had swimbladder disease.
The white sacs you are describing sound like fungus.
I recommend a good syphoning of the substrate and cleaning the decor and the glass. This will help get rid of the fungus.

Hope this helps
Chloe.