gutted 2 fish dead in 2 days help

[rubyjess]

hi all
im gutted i lost a bentos tetra on thursday night ,then to my horror got home from work today to find my betta on the sand barely alive it had
white fluff on its eyes and one gill,strange as the night before he was feeding fine swimming around fine

 

[mudraker]

How long have you had the fish?

What are the water parameters? Temperature?

Is the tank cycled?

But most importantly: How often have you changed the water?

Sounds like a fungus, which are caused by a buildup of nitrates in the water.