Clamped fins and tails can be caused by poor water quality, bacterial or protozoan infections. Normally it is from poor water quality that encourages protozoans to flourish in the water and they do the damage.
Female livebearers (including guppies), do sometimes die during labour, however the clamped tail would suggest something else, or possibly a contributing factor.
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I would recommend bigger water changes done more frequently. If possible do a 50-75% water change and gravel clean the substrate each week or at least once a fortnight (preferably 75% each week). Fish live in a soup of microscopic organisms ranging from bacteria, fungus, protozoans, flukes, and various other things. The big water changes help to dilute nutrients and disease organisms in the water making it a healthier environment for the fish.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
When you do water changes make sure you use buckets specifically for the fish. If possible get a couple of new buckets and use a permanent marker to write "FISH ONLY" on them. Keep those buckets with the fish keeping gear and don't let anyone use them for anything else.
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Filters should be cleaned at least once a month and every 2 weeks is great
When you clean the filter materials, you should wash them in a bucket of tank water. When they are clean, wash them in a second bucket of tank water and then put the filter materials in the aquarium or a bucket of clean tank water.
Wash the filter case and motor under tap water, then put the filter materials back in the filter, fill it up with tank water and turn it back on.
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Tanks kept in science labs sometimes get cleaning products on them (from the cleaners). It is a good idea to put a sign on the tank addressed to the cleaners saying "Do not touch aquarium and do not spray anything on or near the aquarium".
This should help prevent the cleaners from using various products on the tank and possibly poisoning the fish.
Cleaning products are not an issue with the dead guppy, I am simply mentioning this because I have seen aquariums in schools and various other places that have been sprayed or contaminated by the cleaners.
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If any more fish get clamped fins, the best thing to do is a big water change and gravel clean the substrate, then add some salt (sodium chloride) that does not have any anti caking agents in. You can add rock salt (often sold as aquarium salt), sea 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.
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 2 heaped tablespoons per 20 litres and if there is no improvement after 48 hours, then increase it so there is a total of 4 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, 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 will affect some plants. The lower dose rate will not affect plants.
After you use salt and the fish have recovered, you do a 10% water change each day for a week. Then do a 20% water change each day for a week. Then you can do bigger water changes after that.
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To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
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There's more information about what to do if a fish gets sick at the following link. It is similar to what I have written above but more in depth. It's long and boring but might be worth a read if you have some spare time. I suggest printing it out and reading it in bed to fall asleep.
http://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/