Chinese Sucker fish is sick!

Question

Chinese Suckerfish   4 fish in the 5 gallon tank, including the 3 yr old Chinese Sucker. The other 3 fish are small neon tetra type fish. Temp is 75 degrees (fluctuates btwn 70-79 degrees). I changed the filter on Monday. Sucker fish is skinny but still tries to eat. He is weak & struggling to not be upsidown. About 6-8 wks ago, we did a cleaning of the tank. I added 3 plants & bought him new food pellets (Hikari Tropical sinking wafers, for bottom feeders, contains spirulina, silkworm & krill).  Also on Monday, I changed out 1.5 gallons of water. He has been sick for about 2 weeks & today he seems worse.  I think he might die. Help!

Answer
I going to be slightly mean and absolutely blunt.

A Chinese Algae Fish can reach the maximum size of 12" it hasn't grown in 3 years because it is STUNTED. It is considered cruelty to keep a large creature in that tank size. They need the MINIMUM of a 55 gallon aquarium. He's probably meeting his death because of his long slow death of stunting.

What is stunting?

Stunting is when their internal body (organs etcetera) grows and grows but their physical body does not. So imagine your heart and lungs growing bigger but you are still the size of an infant.

I would honestly consider euthanizing the poor thing with clove oil. It will never grow bigger and it probably is in pain.

The neon tetra's need 6+ schools and a 20 gallon aquarium by the way.

You should never clean the filter cartridge, your tank goes into mini-cycles, and it unbalances the aquarium.

Consider restocking the entire aquarium and PLEASE do some research before you take care of any fish the next time. A 5 gallon aquarium can only hold a single Betta Splenden as that was what they were created for.