Guppies keep clamping

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IvyFangs

Aquarium Advice Newbie
Joined
Mar 26, 2025
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2
Location
england
Hey everyone! Over the past couple of months I’ve had about 3 fish get clamped fins and die as well as another 3 go completely missing! I have no clue how they’ve disappeared because there’s no bodies in the tank.

My tank is 160L, has 1 pearl gourami, 2 amano shrimp, 5 Pygmy corys, 7 peppered corys, 1 lil clown plec, 2 honey gouramis and then the remaining 5 guppies :( Nothing in there is particularly aggressive and I’ve never seen any chasing- other than the pearl after the male gourami one or two times but they dont ever fight at feeding time.

Here’s some pictures of the ones that have died that I took. The one that’s white I’m considering that he could’ve been attacked but I’m not sure if fin clamping causes fin tears like that as it looks very different from the previous ones that died.

I took my water for a test today before I did a water change and everything was perfect besides a little bit of nitrates but it was a tiiiiny amount. I do have some test strips that I was using but I know they can be temperamental. However it did say the alkalinity was like mega high, I’ve been using Seachems Acid Buffer to bring down my PH from something stupid like 9 to the 7.5 but I would’ve thought that should have done something about the alkalinity as it’s meant to convert it into CO2? I didn’t want to add too much in case it lowered the PH too much.

Anyone have any ideas?? How can I get the alkalinity down if it is that? I’m gonna be ordering some tests soon.
 

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Do you have anything in the tank that is calcium based (i.e. Crushed coral or seashells or limestone)? Is the alkalinity in your tap water also very high? Clamped fins on Guppies is usually a water issue if over time or parasitic infestations on new arrivals. This is why quarantining new fish is so important these days. The missing fish were probably consumed by the cories and pleco. A Pleco can make a dead fish disappear rather quickly. Let's start here. (y)
 
Do you have anything in the tank that is calcium based (i.e. Crushed coral or seashells or limestone)? Is the alkalinity in your tap water also very high? Clamped fins on Guppies is usually a water issue if over time or parasitic infestations on new arrivals. This is why quarantining new fish is so important these days. The missing fish were probably consumed by the cories and pleco. A Pleco can make a dead fish disappear rather quickly. Let's start here. (y)
Thanks so so much for the response, nothing calcium based I’m afraid but it’s likely that it’ll be our water. We’ve never had it tested but we have water from something called a borehole which is essentially a well. It’s completely untreated other than filtered but we have issues with limescale so it’s likely that calcium is the issue I’m considering stealing water from my boyfriends house and just using that for water changes as he has normal people water haha.

Another issue that I forgot to put in is that my cat occasionally will hit the glass with her paw but it doesn’t seem to be a stress issue as they don’t really care if she’s right up against the glass when she has been tapping at it, if anything they’ll come up to her.
 
If your pH and alkalinity are coming from your tap water, and if you are suffering from limescale issues then it probably is doing, I would look at an alternative way of lowering these than adding chemicals.

pH is a measure of how concentrated hydrogen ions are in a liquid. The more hydrogen ions, the lower the pH. If you add something that increases the H+ ions it lowers the pH, if you add something that decreases the H+ ions then it increases the pH. The scale is logarithmic, so a pH of 8 has 1/10 the concentration of H+ than 7, a pH of 9 has 1/10 the concentration of H+ than 8, etc. Alkalinity is sometimes called buffering capacity because it's absorbs acidity and keeps your pH stable. The product you have is a mild acid, you add this and to start with your alkalinity just absorbs the acid in the product. If you have a lot of alkalinity to overcome, you either need to add a lot of these products or use a stronger acid.

Most of the time chemically altering aquarium water causes more issues than it solves. The best way to address your alkalinity isn't to add chemicals, but to dilute the alkalinity. You do this by using RO water which is water that has had all the mineral content filtered out, so it essentially has zero alkalinity. A 50/50 mix of your tap water and RO water should result in a halving of the alkalinity. You can adjust the mix proportion to get what you want to achieve.

Give your boyfriends water supply a try see if that improved things, that's a more cost effective solution, but getting a 3 stage RO filter would be a solution to your alkalinity issue, and for a little more expense a 5 stage RO/DI filter would also give you a supply of much better quality drinking water. I wouldn't be drinking water at the parameters you describe.
 
Aiken gave the same answer I would have. You need to either cut your water with another better source (i.e. RO, store bought distilled or spring water) or use a whole other source for water. Fish like consistency so if you are constantly adjusting your water with the fish in it, it will make them worse off.

I'll also add that back in the day, we used to have big water problems in Florida so it was common to have a spare barrel of water that was premade to the specifications we needed so water changes weren't a big issue. If you go the using the boyfriend's water route, you may want to get yourself a container to hold enough water to do a number of water changes so you have it handy and in case of emergencies. (y)
 
I don’t have much more to add than what Aiken and Andy posted here. The solution is simple - cut water with RO or some other source to bring down alkalinity. Just make sure it is repeatable each time you have to replace water in the tank. I Personally have used this specific product for the last four years.


It gives 0 PPM water that I then re-mineralize. My tank is smaller than yours so it’s easier.

Lastly, whatever you do make sure you do it slowly and not cause a sudden change in water parameters. This can lead to shock and also hurt your other remaining fish.

Good luck.
 
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