New Member
Jun 22, 2020
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Los Angeles
Firstly, I’d like to say I’m a bit new to fishkeeping, but I’m learning every day — thanks in advance for understanding.

[EDIT] Picture attached is rotated — he’s always swimming horizontally. That’s just him pecking at some algae.

I currently have a 20 gallon freshwater tank with some cardinal tetras, rasboras, and all male guppies (plus a couple of cherry shrimp to help with the algae). I have 2 marimos and cholla wood as well. Aside from 2 rasbora casualties, all fish have been alive and doing well for the past 3 weeks.

BUT There’s one young guppy in there that as been having clamped fins for about 2 weeks now. When he was introduced to the tank, he was fine, fins were fanned out. He’s a little strange — he eats fine, swims fine, but exhibits strange behavior. None of the fish pick on him, and he seems to like either hanging by himself or schooling up with the tetras.

Checked my water levels:
- Nitrates: 15-20 (Safe)
- Nitrites: 1-2 (in caution to stress levels)
- Hardness: 300 (pretty hard)
- Chlorine: 0 (Safe)
- Alkalinity: 120-130 (ideal)
- pH: 7.6-7.8

I’ll be looking into how to reduce nitrites, but it seems like all the fish are doing fine except a young guppy I have in here which, like I said, is having clamped fins... In the case that it’s parasites, I’ve been putting doses of Paraguard in the tank everyday. I also do a 20-40% water change every week with water conditioners. I’ve also installed an air pump with air stone in case the tank isn’t getting enough oxygen.

Kind of stumped at this point since every fish is doing fine other than this little guy. :( Any help would be appreciated!


Fish Guru
Jan 26, 2008
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Perth, WA
The picture didn't work.

Any idea what the ammonia level is?
How long has the tank been set up for?
How long have you had fish in the tank for?
How often and how do you clean the filter?

The easiest way to reduce ammonia, nitrite or nitrate is by doing a 75% water change and gravel cleaning the substrate every day until the levels reach 0ppm. This is also what I recommend for sick fish.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Wipe the inside of the glass down with a clean fish sponge.

If the filter has been running for more than 2 months, clean it if it hasn't been done in the last 2 weeks. Wash the filter media/ materials in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn.

Do a 75% water change and gravel clean the substrate every day for at least 1 week.

Add 2 heaped tablespoons of salt for every 20 litres (5 gallons) of tank water. Keep the salt in the tank for 2 weeks.
When you do the daily water changes, add salt to the new water before adding it to the tank so the salinity in the aquarium remains stable.


Fish Aficionado
Apr 16, 2020
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Bristol, UK
Water hardness is perfect for live bearers, but its too hard for any kind of tetras as they need softwater.


Staff member
Global Moderator
Nov 28, 2006
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Teesside, UK
You don't give a reading for ammonia (test strips don't include it so you need another tester for ammonia). With nitrite in the water you probably have, or had, ammonia as well. Both of these in the water will stress fish, and clamped fins is a classic sign of stressed fish.

You say the fish have been in the tank for 3 weeks - did you cycle it first by adding ammonia to the water to grow the two colonies of bacteria? If you didn't, you are doing a fish-in cycle and this means a lot of water changes to keep the fish safe.
Both ammonia and nitrite are toxic for fish. Ignore the instruction leaflet - any reading above zero for ammonia or nitrite is bad. As Colin_T says, you need to do water changes to reduce nitrite to zero. Test every day and whenever it is over zero, do a water change.

As a side issue, Salty&Onion is quite right. With a hardness that high, your soft water fish (cardinal tetras and rasboras) will not do well. They are likely to not live as long as they should. Can I suggest that if you need to replace them, choose hard water fish instead of soft water fish.
Research fish on https://www.seriouslyfish.com/knowledge-base/ This will tells you the hardness, tank size etc needed by fish species. Your hardness of 300 is ppm; that converts to 17 dH, the other unit used in fish profiles.

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