Hmmm...tell her these facts...
Clown Loach - Grow up to 14" and should be kept in groups of 4 or more. They should also live for at least 20 years! They need a 75gal tank to grow properly.
Bala Shark - Grow up to 14" and should be kept in groups of 4 or more. These should also live for at least 15years. They need a 6ft tank of at least 100gal to allow them to grow to their full size. They are very active fish and need the room to swim aswell as grow.
Discus - How long has it been in the tank? Discus are notoriously hard to keep. I would expect it to be dead within 6months in those conditions. They should live to around 10 years old.
The fish above are the reason that the tank has green water.
Not only is she killing them, but she wont be able to see her fish unless she gets rid of the unsuitable ones.
And it's not that the fish 'could' die. They WILL die of premature deaths.
They only show symptoms around 24-48 hours before they die. Until then they will swim around looking and acting normally as the permenant damage is done to their gills and nervous system.
Then one day they will start gasping for air, perhaps encounter swim bladder issues and will then die.
And if it isn't the ammonia poisoning that gets them, they it will be the stunted growth. Their organs will still grow at the normal rate even though their bodies do not. This causes internal pressure which can cause swim bladder issues aswell as a slow painful death.
At the end of the day they ARE her fish. And what she decides to do is up to her.
But just make sure she knows the facts.
When they die prematurely(at only a couple of years old as opposed to over 15!), it will be directly down to her.
This geinuinely makes me a bit

but what can I do.