What is the ammonia, nitrite and nitrate level in the water?
What sort of filter do you have and how do you clean it?
Established biological filters should be cleaned at least once a month. You are doing it every 2 weeks and that is fine. However, if the filter is less than 6 weeks old, do not clean it. When you do clean the filter, wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. The filter case and motor can be rinsed under tap water.
If you are replacing the filter media when you clean the filter, you will be getting rid of the beneficial filter bacteria and could have an ammonia or nitrite reading, and this could kill the fish. Any ammonia is bad for fish and if the pH is above 7.0, it becomes more toxic. You appear to have some limestone rocks in the tank, which are made from calcium carbonate and that will raise the pH up to around 8.5, maybe a bit less. If the pH is this high, a very low level of ammonia could kill the fish.
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What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website (Water Analysis Report) or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
Mollies need a GH of 250ppm or higher, and a pH above 7.0.
Bettas and bristlenose catfish come from water with a GH below 150ppm and a pH below 7.0.
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What sort of fish have been dying?
What symptoms do they show?
How soon after the water change or filter clean do they die?
I can see a bristlenose catfish, a Betta and some balloon mollies.
What other fishes are in the tank?
Balloon mollies are badly inbred and severely deformed to have a much shorted body than their ancestors. This causes their internal organs to get squished up and the organs can't always work normally. This along with the genetic issues caused by inbreeding makes balloon fishes very weak and most of them die within their first year. It's preferable to avoid balloon fishes of any kind due to these issues.
Mollies from Asian fish farms regularly carry intestinal worms and gill flukes. These weaken the fish and make them more susceptible to diseases. You can't normally tell if the fish have worms but 99% of the livebearer fishes (mollies, guppies, swordtails, platies) coming out of Asian fish farms have them.
Section 3 of the following link has information on treating fish for intestinal worms. Round worms (Camallanus) are the most common found in aquarium fish and can be treated with Levamisole or Flubendazole.
Fish do a stringy white poop for several reasons. 1) Internal Bacterial Infections causes the fish to stop eating, swell up like a balloon, breath heavily at the surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be...
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