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 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).
Virtually all rainbowfish do best in hard water with a high pH. The only exception to this is wild caught Iriatherina werneri, Rhadinocentrus ornatus and some of the Pseudomugils that come from soft water. But Melanotaenias and Glossolepis all do best in hard water.
I had mine in tanks with a GH above 200ppm and a pH above 7.5.
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FOOD
What are you feeding them?
Rainbowfish need at least 50% of their diet to be plant based. If they don't have enough plant matter in their diet, they have a lot more health issues.
You can use aquarium plants like Duckweed, Ambulia, Water Sprite and any type of green algae to supplement their diet. Use goldfish foods or vege flakes/ pellets in addition to other foods. You can add marine algae and various fruits & veges too.
Avoid frozen bloodworms. I had nothing but problems when I used frozen bloodworms with my rainbows. Other people have used bloodworms and not had issues, but if you are having problems with the fish and you feed them frozen bloodworms, drop it for 6 months and see if it helps.
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FISH TB
Have any of the fish that died shown the following symptoms?
- small red sores on the body that open up into ulcers
- sudden bloating up
- stop eating
- do stringy white poop
- gasp near the surface or near the filter outlet
If yes, you should get them tested for fish TB. The above symptoms are commonly seen in fish that have a Mycobacterium infection and it will go through an aquarium and kill one fish here and one fish there over a period of years.
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NOT EATING
If the fish stop eating but don't swell up and try to eat but spit it out, that can be an infection in the mouth, gills or sinuses. I used to treat this problem with big water changes and salt.
Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.
Do a 75% water change and gravel clean the substrate. 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. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter. 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. Cleaning the filter means less gunk and cleaner water with fewer pathogens.
Increase surface turbulence/ aeration when using salt or medications because they reduce the dissolved oxygen in the water.
Add some salt, (see directions below).
It's also a good idea to deworm rainbowfish because they are frequently infected with intestinal worms like Camallanus.
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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 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, 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.
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.