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I have had a number of goldfish (three pairs of two) over the last six months or so. In all cases they have lasted about 2 months before dying. I have no idea what I'm doing wrong so here is all (hopefully) the necessary info, if anyone has any idea what I'm doing wrong please let me know.

  • The tank is 25 litres (I know it'll need upgrading when they get bigger but they never get that far).
  • There are three plants in there with a plastic log for the fish to hide in (I used to use a wooden log but I switched to a plastic one just in case the wood was causing a problem). There is gravel at the bottom of the tank.
  • The pump is a Marina i25 and the cartridges are replaced every two weeks or so. I used to use the pump that came with the tank but upgraded to make sure it wasn't the cause of the problems.
  • I do a partial water change every week/10 days:
    • I siphon some water into a bowl.
    • I use a net to get the fish out and put them in the bowl.
    • I clean the glass with a microfibre cloth, and clean the pump (replacing the cartridge if necessary).
    • I siphon out about a third/half of the water into a bucket and discard it.
    • I siphon enough water to top the tank back up. This water has been dechlorinated with Tap Safe and left in the bucket for 24 hours to come to the same temperature of the water in the tank.
    • I leave the tank to settle for half an hour or so and then put the fish back in, discarding the water from the bowl they were temporarily in.
  • They get fed with coldwater fish food every other day. I give then a pinch of food and they will almost always eat it all. A couple of times a week I feed them shelled peas - they seem to love them!
  • The fish aren't always from the same shop (or even the same chain). I've had some fancy ones but resorted to the bog standard "hardy" goldfish in an attempt to make them last longer.
  • The water has been regularly tested at the local pet shop and they have never found anything abnormal about the levels.
  • In the past I have tried to treat them with salt but all this does is kill the plants, the fish don't get any better.
  • After one pair of fish die the tank is emptied and cleaned (just with warm water) and refilled before adding more fish.

I know this is a very general question but I would appreciate it if anyone has any advice, it is slightly disheartening that I seem to manage to regularly kill off pets that are supposed to be very easy to look after. I feel like there must be something obvious that I'm doing wrong.

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    For those that don't want to look up conversions. 25 liters is about 6.6 gallons. – Spidercat Nov 23 '15 at 23:09
  • Without being offensive, do yourself and the fish a favour and either get a >150L aquarium with at least 2 external bio filter or stop it. Foremost, read about "aquarium water basics" :) - the bigger the tank the less stress for oyu and the fish. – Aurigae Dec 13 '15 at 6:07
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Your tank is way, way too small. Fancy goldfish need about 20 gallons (about 75 liters?) for the first fish, and 15 gallons (56 liters) minimum for every fish you add after that--so a pair of fancies needs at least 35 gallons, etc. I'm not sure what you mean by "hardy" goldfish, but I'm thinking you mean comets, which grow too big to be housed in most aquariums and need a pond at the very least. Goldfish also need to be fed every day, about two or three times a day, as much as they'll eat in about one or two minutes, and then you scoop out the leftovers to prevent the food from rotting in your tank.

You don't mention if you've cycled your tank or not, but that might be worth looking into if you're not sure, and I'd invest in a testing kit of your own (liquid testing kits tend to be more accurate than strip tests, which is what most pet stores use). Upgrade to a much larger tank if you're still interested in goldfish, but the size you have now is perfectly fine for a betta if you'd rather go down that route. Don't feel discouraged, goldfish are actually much harder to care for than most people are lead to believe.

  • Sounds like some good advice there. Thank you. Is that size guide true for small goldfish (around 5cm long)? – Phil Nov 23 '15 at 20:04
  • After a bit of a search on Wikipedia it looks like I've had a "common goldfish" in the past. I'll look into betta. – Phil Nov 23 '15 at 20:08
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    On cycling a fish tank: pets.stackexchange.com/questions/215/… – Spidercat Nov 23 '15 at 23:08
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Get yourself a fresh water liquid test kit and test the Ammonia, Nitrates and Nitrites. I wouldn't trust the pet store tests. They said our water was fine but actually we had toxic levels of Nitrites, I think they may have only tested the Ammonia. The test should also be performed just before a water change as this is the point at which your levels will be the highest/most toxic.

Goldfish can live for 20 years so having them die after only two months indicates that there is definitely something wrong.

Ammonia

If you have more then zero Ammonia then your tank isn't cycled and this is the reason your fish are dieing. To fix this you either need more bio media in your filter (or a bigger filter) or to reduce Ammonia sources (less fish or less feeding or a bigger tank)

Nitrites

If you have more then zero Nitrites then this is what is killing your fish. This fix for this is the same as for Ammonia.

Nitrates

If you have more then 50 ppm Nitrates (before a water change) then this could be what is killing your fish. To fix this you need to do more frequent water changes or get a larger tank or reduce the amount of fish. A filter can't remove Nitrates so getting a bigger filter with more bio media won't help.

Filter

Your filter could be the problem. When you replace the cartridge does this include the bio media (thats what it looks like from the link). If so then your tank is definitely not cycled. It takes 4-6 weeks for your tank to cycle. (beneficial bacteria needs to build up on your bio media to a level sufficient to convert Ammonia to Nitrites and Nitrites to Nitrates) This takes 4 to 6 weeks. If you are replacing the bio media every two weeks then this process can never complete and will start again from scratch. Your Ammonia and Nitrite levels will be out of control and your fish will die.

All your filter really needs is a sponge/filter foam (for mechanical filtration) this can be rinsed out in aquarium water, and so doesn't need to be replaced. And some bio media (which shouldn't be replaced as described above). The carbon is only really required to remove medication from your tank. Lots of experienced fish keepers don't use carbon at all. So they are really onto a money winner with you by getting you to replace your filter every two weeks.

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