Help with overcrowding

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Skobi

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Hi all, I have a 75 gallon tank that I intended to be a community tank. I started out with 6 platies and 6 guppies all female and a pleco. Well the female guppies ended up being pregnant when I bought them so now I have an overcrowded tank of probably 120 guppies, only 2 platies left and I don't know what to do. I just pulled 8 dead guppies out today and I assume they are dying due to the overcrowding. I was going to sell some back to my LFS but due to covid they aren't buying right now. I contacted the LFS when I first noticed fry in the tank but they told me guppies notoriously eat their fry and overcrowding wouldn't become an issue, they were wrong. Help please?
 
Pictures of the sick fish?

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If your fish ever get sick, do the following:
Wipe the inside of the glass with a clean fish sponge.

Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media/ materials in a bucket of tank water and re-use them.

Do a 75% water change and gravel clean the substrate every day until we work out what is wrong with the fish.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Increase aeration/ surface turbulence to maximise oxygen levels in the water.

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Add some salt. See directions below.
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 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, 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 will not affect 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.
 
I didn't think to take pics and have already disposed of them, but they looked completely normal aside from being dead. They weren't bloated, no sign of fin rot or anything like that. They did have some wounds though which is why I thought their deaths were directly related to overcrowding, like there isn't enough space so they are killing each other. Everything else in the tank is great. I have 3 filters and I only change one per week so as not to remove the good bacteria, the ammonia, nitrate and nitrite are all where they need to be and the water is crystal clear not cloudy. I just don't know how to reduce the number of fish or keep it from increasing.
 
Nevermind, I checked the tank again and had 4 more dead guppies and noticed all the remaining fish were staying at the top of the tank and the pleco was swimming around frantically. So I pulled out a new bottle of the ammonia test and rechecked and the ammonia level was off the charts. I did an emergency water change, threw out the old bottle of testing fluid and added some live bacteria. I will test again in a couple hours and see where it is at.
 
I contacted the LFS when I first noticed fry in the tank but they told me guppies notoriously eat their fry and overcrowding wouldn't become an issue, they were wrong. Help please?
There are guppies that will eat their fry but there are also a lot of guppies that will leave the fry alone. So, I wouldn't say that guppies are notorious fry eaters as a general remark. To me it means that at the lfs they just don't know what they're talking about.
 

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