I usually change the water within 3-4 weeks, but since I just added a bunch of fish, I will probably start doing a water change once every other week or so.
I clean and / or change the filter once a month at least, I usually check if it's getting clogged.
I don't recall the dimensions, it's a fairly standard 55 gallon long.
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So my main focus is just if I could add 6 more rainbowfish or if that seems too much.
And I am concerned about times that fish have randomly died. Like I said my oldest turquoise rainbowfish died recently, and for 2 or so years I never saw one problem with him at all.
That morning I just got up, and noticed he was upside-down, still alive and struggling. And I know that's usually a swim bladder issue, but why would that happen out of the clear blue? He was totally perfectly normal for over 2 years, and then one day he's just struggling and later that night he died.
How much water do you normally change?
What are the ammonia, nitrite and nitrate levels?
What is the pH and GH of the water?
Rainbowfish don't tolerate high nitrate levels and you want to keep this below 20ppm.
Rainbowfish do best in water with a pH above 7.0 anda GH above 200ppm. Melanotaenia boesemani come from hard water with a GH above 300ppm.
Black neon tetras come from water with a pH below 7.0 and a GH below 100ppm.
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Do you feed the rainbows much plant matter?
Rainbowfish need about half of their food to be plant based. You can use vege flakes/ pellets, marine algae, or aquatic plants like Duckweed.
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Do you actually throw filter media/ materials away and replace them with new media, or do you just clean the media in a bucket of tank water and re-use it?
Filter media should only be replaced when it starts to fall apart. If you have cartridges in the filter, you can replace these with sponges that will last 10+ years. If you replace media, only do one piece at a time and wait a month before replacing another piece.
If you have carbon or Ammogon/ Zeolite in the filter, remove these and don't replace them. Just put a sponge in their place.
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I live in Australia and we sell tanks by the foot (3ft, 4ft, 6ft), and subsequently have no idea how big a 55 gallon tank is.
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I wouldn't add any new fish until you work out why they are dying.
Did you do anything to the tank on the day or the day before it died?
Unfortunately, rainbowfish regularly have Fish Tuberculosis (TB) and can live with if for a few years. The bacteria slowly grow in/ on an internal organ. One day the bacteria damage the organ and it fails. When this happens, the fish usually stops eating, bloats/ swells up, gasps or breathes heavily at the surface or near a filter outlet, does a stringy white poop and dies a few hours later.
I'm not saying the fish died from TB but it should not be ruled out.
I would monitor the remaining fish over the next 6 months and see if any more die in a similar manor. If they do, take a body to a fish vet and have them necropsy (animal autopsy) the fish to see if it has TB.
In the mean time, avoid putting your hands in the tank if you have any cuts or scratches on them. If the fish have TB and you have an open wound and get fish water on it, you could get a localised TB infection in the wound called a granuloma. These need to be biopsied at a doctor and samples sent away for a positive identification, then you get put on anti-biotics for a while. So try to avoid getting fish water on you if you have cuts or scratches, and wash your hands with warm soapy water after working in the tank.
You can wear rubber gloves when working in the tank if you are concerned. Just make sure they are free of chemicals and only used for the fish tank.
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If your tank is 4 foot or longer, it is big enough for any species of rainbowfish. Most only grow to 4 inches or less. If you have them in a huge tank (20 foot long x 3 foot high) and keep them long enough, they might reach 5 inches but they don't normally get that big. I had rainbows in ponds that were 10 foot in diameter and 3 foot deep/ high, and the male rainbows hit 5 inches and that included their tail. The females got to about 4 inches, including their tail.
