My goldfish was showing signs of illness, then started shredding it tail, loosing its colour - I did a compete water change and cleaned the filter and tank/gravel. purchased some tri sulfa tablets. Once I added these into the tank the fish sunk to the bottom, very little movement - after 20 mins it did two large thick poos. Is this normal?
Treat the fish for Ammonia poisoning
If it's been any length of time since you took all the gravel, filter, water out and replaced them, you need to do two things:
- Check the ammonia levels in your tank
- Treat the tank if the ammonia levels are high
By washing your filter and gravel in hot water, the chances are you've killed all of the beneficial bacteria that live in the substrate (gravel) and the filter (in the sponges / filter media) which means that there is nothing left to convert ammonia into nitrite and nitrate. You'll probably want to learn about the nitrogen cycle here (video).
Your fish may have been suffering from ammonia poisoning to start with. Remember that any amount of ammonia in the water is potentially lethal to fish. You should avoid doing 100% water changes because it is normally not necessary and if the parameters of the water has changed at all then your fish will have to adapt to it very quickly which can be stressful. There's a lot of debate about how much water you should change but normally it is somewhere between 20 - 40% per week.
- Check your water parameters with a test kit
- Consider changing your maintenance routine.
- Add some 'quick start' aquarium products to boost the beneficial bacteria levels in the tank.
- Be very careful removing all the gravel or filter material because these contain the bacteria which is the life blood of the aquarium.