The fish has an internal problem. It is skinny and if it doesn't eat much and does a stringy white poop, it has an internal protozoan infection.
If the fish had bloated up overnight, stopped eating and does stringy white poop, it is an internal bacterial infection.
If the fish was skinny but eating normally and doing a stringy white poop, I would say it has intestinal worms.
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He has been swimming in downward spirals for the past hour
If it has started to spiral through the water, then euthanise it.
When fish spiral through the water is it usually caused by a bacterial or protozoan infection in the brain and there is no cure. The fish usually die a few hours after they start doing this.
The spiralling through the water combined with stringy white poop, skinny fish and not eating would suggest it has a protozoan infection. These are common in tanks that have a lot of rotting matter in them or don't get enough water changes. You can usually stop it spreading to other fish by cleaning the tank conditions up and adding salt, (see instructions below).
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You also need to increase the GH of your water. Mollies need a GH above 250ppm and a pH above 7.0. your pH is fine but the GH is way too low.
You can use a Rift Lake conditioner to increase the GH, KH and pH. It is available from pet shops or online and is used to increase the GH, KH and pH of water for African Rift Lake cichlids.
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Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate every day for a week. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn.
Increase surface turbulence/ aeration when using salt or medications because they reduce the dissolved oxygen in the water.
Add salt, (see directions below).
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SALT
You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 2 heaped tablespoon per 20 litres (5 gallons) of water.
If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 4 heaped tablespoons per 20 litres.
Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, Bettas & gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.
The salt will not affect the beneficial filter bacteria but the higher dose rate (4 heaped tablespoons per 20 litres) will affect some plants and some snails. The lower dose rate (1-2 heaped tablespoons per 20 litres) will not affect fish, plants, shrimp or snails.
After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.
When you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.
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After you have finished using salt, I would deworm the fish just to make sure they are free of internal worms.
The following link has information about stringy white poop and includes treating intestinal worms in fish (post #1, step 3).
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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