Fish sickness usually doesn't have a direct correlation to plants.
It usually has to do with either bacterial, or parasitic problems.
And the severity often has to do with toxic conditions,
Sick fish and sad looking plants may be symptoms of something else.
What is the frequency and amount changed in your water change routine?
How often and how do you clean filters?
Water are your water parameters? (nitrate, pH, ammonia)
How many fish in what size tank?
Do you occasionally add new fish without quarantining them?
The above, are all questions that need to be answered before diagnosing that type problem