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New tank syndrome? What to test next?

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My brother recently bought a small 25L tank and after scaping it and running the filter for a few days got a few small killifish to cycle it.

2 of them have since died (the first we put down to stress or just potentially bad luck) but the others seemed to be happy. However a second followed a few days later and now the third isn't looking very happy (luminent parts dulled, hiding up the back not moving a lot). I get the feeling this last one might be a lost cause, but before we try and add any more fish we obviously need to identify the issue so this doesn't continue. At this stage I don't think the tank is fully cycled, but would have thought 3 small (1cm tops) fish wouldn't have been overly taxing.

Details below

Problem:- Fish dying, sequentially a few days apart, seem fine until they lose colour, hide then eventually die (and we've not been able to find them yet)

Ph:- 7.4 - 7.2 when testing

Ammonia:- 0.25ppm

Nitrate:- about 5 (according to kit, could be higher)

Nitrite:- 10-20ppm

Gh:- Not tested

Kh:- Not tested

Size of tank:-30cm cube, 25L

Temperature °C:- 24

Been running for:- A week and a bit

Filtration:-Canister/Sump/Internal Filter/Sponge:- Hang on back Aqua clear 20

Fish in tank:- 3 (now 1) killifish

Plants in Tank:- Some val, some rotala and some lileopsis (sorry spelling) - Fertilizer put in once a week. DIY CO2 (Note: Maybe this is a problem?)

Feeding:- What food and How often: Micropellets, once a day. Visibly watching fish eat them, no more than a tiny pinch (5-10 granules)

Recent Medication Treatments:- None

Last water change:- 5 days ago

Water change every Day/Weekly/Monthly: - Weekly so far, but an extra water change when the nitrite started to creep up.

Any thoughts? Are these just the wrong fish (not enough in a school)? Should we test for more chemicals? Temp ok? Just too much bioload to start? Any other pollutants?

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c02 could be the issue here but it can also be compounding the amm nitrite and nitrate. the problem i tend to find with my 20l tank is that it takes next to no waste to start showing up in tests because with such a small environment it acheives a greater ppm because of the small literage. do you have a co2 meter?

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I'm thinking having looked into it a bit more that nitrite poisoning was probably the story.

I had (mistakenly) said to watch the ammonia and nitrates which were the issue, and I dont think we acted on nitrites enough.

So advice I'm getting is - don't use c02 until later, dont go for the killifish to start, and since we've got such high nitrites, just wait for the tank to cycle before thinking about a new fish?

Good plan?

Thanks everyone for your quick response

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