Fin rot, stress or nipping?

Hello. So I have a 20 gal hex freshwater tank. I cycled it with some gravel and a cave from my old tank for 3 weeks. A couple of days ago I added 2 platies. The man at the aquarium store said it’s help cycle the tank and these fish were hardy. They seem to be adjusting fine to the water, I added them in slowly when I got them home. Anyway, the next day the orange one lost half his tail. It was ripped and the remaining half of his tail fin was dangling by a thread. I thought the other but him but I’ve been carefully observing and there is zero aggression between them. So..what the heck could make his fin rip off? It was gone in 4 hours . Now the last couple of days he’s not eating. I’m watching my water and the ammonia is only at 10ppm, nitrates and nitrites are low. Water is clear. I have false plants and a few decorations. Maybe the coral got him? I’m so stumped and was hoping you could help. I’m hesitant to add any fin rot antibiotics as a precaution because I don’t want to ruin my cycling. Any help is appreciated.

Can you recheck your test results and post them here in numbers? If ammonia or nitrite are above zero, do a large (50-75%) water change using a dechlorinator straight away. Ammonia if 10ppm would be fatal.
Please can you also post a picture of the fish and the tank?
My initial thoughts are an injury because of the speed that It happened but we need the information requested.
You are right to avoid medication without a diagnosis.

I agree with above post. Additionally, knowing your water parameters would help, as livebearers must have moderately hard water to last [this is not the immediate issue now, but it may be a contributing factor]. Parameters refers to GH (general or total hardness), KH (carbonate hardness or Alkalinity), and pH [temperature is also a parameter but one you can control so not directly related to the source water params]. You should be able to ascertain these from your municipal water authority, check their website.

I am inclined to think ammonia is the culprit if it really is as high as you say, this can rapidly burn fish. But let's get all the data.

And, welcome to TFF.

Naughts said:

Can you recheck your test results and post them here in numbers? If ammonia or nitrite are above zero, do a large (50-75%) water change using a dechlorinator straight away. Ammonia if 10ppm would be fatal.
Please can you also post a picture of the fish and the tank?
My initial thoughts are an injury because of the speed that It happened but we need the information requested.
You are right to avoid medication without a diagnosis.

Click to expand...

Apologizes, I had that incorrect. My latest test is: Temp 75 F, Ph 7.6, GH 6, KH 15, NO2- 2ppm, NO3- 10ppm, and NH3- 0.50 ppm. So my ammonia is low, nitrites low, nitrates at 10. I will post a picture with a follow up post when I have my phone handy. His tail fin is gone completely. I'm still cycling, I set up my tank approx 3 weeks ago, and only added the platies a couple of days ago. I figure from my readings I'm nearly done. Just waiting on those nitrates to drop. The 2 platies show no sign of distress aside from the missing tail fins on the orange one. The other is swimming, active, inquisitive, eating, etc. Today, the orange one seems to have perked up and even nibbled some food. Could it be stress from a new home? It's a mystery.

Nitrite at 2 ppm is not low, it is too high. It should be zero. Ammonia should also be zero.
You need to do a water change every time you see a nitrite or ammonia reading that is not zero. These are both toxic and will harm your fish. The tank will be cycled when they both stay at zero.

Nitrate will not drop. It is the end point of the nitrogen cycle and we remove it with weekly water changes once the cycle has finished.


You should also test your tap water. If your water provider uses chloramine to disinfect the water supply, there will be some ammonia in your tap water. And lots of places have nitrate in tap water. You need to know if any of the ammonia or nitrate in your tank water is there because it is in your tap water.

This is the only photo I could post. It said the others were too large even though I tried cropping them. This shows his tail anyway.

essjay said:

Nitrite at 2 ppm is not low, it is too high. It should be zero. Ammonia should also be zero.
You need to do a water change every time you see a nitrite or ammonia reading that is not zero. These are both toxic and will harm your fish. The tank will be cycled when they both stay at zero.

Nitrate will not drop. It is the end point of the nitrogen cycle and we remove it with weekly water changes once the cycle has finished.


You should also test your tap water. If your water provider uses chloramine to disinfect the water supply, there will be some ammonia in your tap water. And lots of places have nitrate in tap water. You need to know if any of the ammonia or nitrate in your tank water is there because it is in your tap water.

Click to expand...

I understand they should be zero but my tank is still cycling. In the article I read, they will drop to zero and nitrates will drop and that's how I know when its finished cycling. I can test my tap water, that's no problem.

Any amount of nitrite in the water is harmful to fish. It does the same to them as carbon monoxide does to us, it stops blood absorbing oxygen. Whenever you see a reading above zero, you really do need to do a water change to get it down to zero. It won't stay at zero very long as the ammonia eating bacteria are constantly making it, so during cycling it will be above zero again next day, so the nitrite eating bacteria will grow even if you do daily water changes. Once there are enough nitrite eaters, they'll eat all the nitrite as soon as it is made and the test reading will stay at zero.

Ammonia is made by the fish, it's part of their waste. The ammonia eating bacteria eat it and their waste is nitrite. Nitrite eating bacteria eat nitrite and their waste is nitrate. But there are no nitrate eating bacteria in our tanks so that just goes up and up. Once ammonia and nitrite stay at zero, we still have to do weekly 50% water changes to remove nitrate, and all the other waste products that we can't test for.

The indicator that the cycle has finished is that both ammonia and nitrite stay at zero. It doesn't matter what the nitrate reading is, the level of nitrate is not an indicator of where the cycle is.

Oh...ok. well thank you for the information. So I should probably do a water change to get my readings down. I was afraid it would mess up my cycle if I did that.

There is a delicate balance between keeping ammonia & nitrite low enough that they don't harm the fish, and high enough so the bacteria grow. I prefer to err on the side of keeping the fish safe even if it means the cycle takes longer.

As for nitrate, there is usually some in tap water. It can be as high as 40 to 50 ppm, especially in areas where the tap water contains run off from agriculture into the river/lake/reservoir - from the nitrate the farmer adds to fertilise his crops. This tap water nitrate gets into the tank when we fill it at the start, and when we do water changes.

As there are fish in the tank and it is still establishing the cycle, this is a situation where a bacterial supplement would definitely help. Tetra's SafeStart is about the best (for this specific situation), but Seachem's Stability will help if that is all you can get locally. The smallest bottle of either is all you need. These products increase the production of the nitrifying bacteria, and they have been proven by scientists to do so.

Another help is a conditioner that detoxifies ammonia and nitrite. There are two I know of, Seachem's Prime and another called Ultimate which Hikari is now marketing. Either is advisable, just remember they detoxify ammonia and nitrite for 24-36 hours, after which it again becomes toxic if still present. Alternate day water changes using one of these, and the bacterial supplement, are advisable.