Hi MichiganMom,
Your niece had made several beginner mistakes. The problem was caused by several reasons.
1. I'd like to know what exactly species that "sucker fish" is. There are plenty of different species with a "sucking" mouth. Many of them are simply way too big to be in a 5 gallon tank. Without knowing the exactly species, it is hard to know for sure.
2. How many mollies were there? The exactly number is important.
3. It is an extension of the first two points. I believe the 5 gallon tank is simply overstocked with
too many fish and unsuitable fish. When you have too many fish or too big of a fish (or both) in a too small of a fish tank, there will be problems such as ammonia poisoning. To make it short, ammonia is a toxic. All fish species produce ammonia on a constant basis through their gills, and the breaking down of their poop and leftover fish food. Basically they are poisoning themselves unless the aquarium has been "
cycled" before the fish were added into the tank.
To understand it further, you need to read on our article of
Aquarium Nitrogen Cycle and Fishless Cycling. It explains everything in details.
It is extremely important that all fish keepers must know.
However, not even aquarium nitrogen cycle will work unless the tank size and the filter GPH is suitable for the fish in there. Otherwise they are basically swimming in their own waste.
P.S. What filter is she using?
4. Was there a
heater?
What was the water temperature? Mollies and many "sucker fish" are basically tropical fish. They can't survive for long in an aquarium without a heater to keep the water at at least 72F+.
5. It is wrong to wash everything in the tank.
All new filters require time to accumulate beneficial bacteria which will feed on the toxic ammonia. <--- You will know this once you have read the article of
Aquarium Nitrogen Cycle and Fishless Cycling. Keep washing the filter will just make the aquarium nitrogen cycle start all over again.
6. It is also wrong to change all of the water at once. It is recommended to not change more than 50~70% of the water. Normally we change only 30~50% of the water once a week as a part of the regular
aquarium maintenance. Too large of a water change will shock the fish with sudden change in water temperature, PH, hardness, etc. Any one of these factors can kill them quickly.
Please tell your niece not to add any more fish. If all her fish had died, it is time to do a fishless cycling before getting any new fish.
