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Jul 25, 2021
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I used to have 9 platies (or platy) in a 70 litre tank with 2 Chinese Algae Eaters, sold as ‘Sucking catfish’. The two catfish and 4 of the platies were rehomed as there were too many males, leaving me with 3 females and 1 male. Soon after, I bought 6 (Golden) White Cloud Mountain Minnows as it is an unheated tank kept around 18 degrees Celsius (room temperature) and I finally invested in live plants. Two weeks later (around 2 weeks ago), one of my female platy passed away, she seemed to be pregnant once again and sinking. Another week later and one other female passed away, she had clamped fins and a sinking tail though always seemed to be skinny. Now, all of the remaining platy seem to be hanging at the bottom of the tank and hiding, though 2 are pregnant and due in about a week or less.
Does anybody know what could’ve harmed the fish? Diseases brought by the live plants or white clouds? Water parameters? Brought from low quality chain stores?

Weekly water changes are done and PH is quite high, around 7.6 last test though I can do it again to make sure. Tank has only been set up for around 3 months. I do not have a test kit for nitrate, nitrite or ammonia yet and am unable to get one as we’re in lockdown and I am a young hobbyist so am unable to go anywhere and my parents don’t agree that the fish are worth the trouble. I will get one in a few weeks though once out of lockdown. Have started to cut down on feeding as I may have been overfeeding slightly. Water also seems to have floating particles, is this normal? How do I get rid of them if not? They float to the top of the water but are pushed down again by the filter current.

Any answers or advice is greatly appreciated.

Do a 75% water change and gravel clean the substrate every day for a week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Add some salt.
You can add rock salt (often sold as aquarium salt) or swimming pool salt to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt 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, 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.

If 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.
Thank you, I’ll get right to it. The male seems to be doing fine so I’m wondering if it’s only because the females are pregnant as I’ve heard they like to hide. Also, I think one of the females that passed may have been a swordtail or swordtail platy hybrid as it looked pretty similar to pictures online. All water is dechlorinated before being added and water changes have been started today. Thank you for the advice, I’ll add aquarium salt ASAP.

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