Mystery guppy deaths

[Audfish7]

I have a tank that's about a month old. I lost two guppies today and it's a bit of a mystery to me. There were four zebra danios, three male guppies, an oops female and two oops fry. Tank parameters are good. 0ppm ammonia, 0ppm nitrates, 5-10 ppm nitrites, 76F. The tank is a 20 long with A HOB rated for this tank size. Water changes weekly, 25%

I noticed what I thought was fin nipping or fin rot on one of the male guppies and started dosing with melafix. One of the other male guppies was half dead last night, couldn't submerge, lying on its side, maybe looking a bit bloated. I euthanized him as it was clear he wouldn't make it. Today another male guppy was dead at the bottom of the tank.

The original fin-damaged guppy is alive. Do you think the danios killed the guppies? I thought that if it was fin rot, the one first showing symptoms would die first. I've only had semi-agressive salt & freshwater tanks, and a goldfish. I wanted a chill tank. If it is aggression, why would they let the slow pregnant female and little fry live and kill the male guppies?

 

[Colin_T]

Hi and welcome to the forum

Do you have pictures of the dead fish?

Need a better picture of the live fish but from the picture provided, the guppy with the damaged tail looks like it has been bitten, probably by the danios, which should be kept in bigger groups (around 10 or more) so they keep each other occupied.

Melafix doesn't do much except make the water smell like a wet Eucalypt forest.

You say your water quality is 0ppm ammonia, 0ppm nitrates, 5-10 ppm nitrites, 76F.
Is the nitrite and nitrate correct? Normally in a healthy tank you have 0 nitrite and a bit of nitrate.
If you have 5-10ppm nitrite, that is bad and probably what killed the fish.

-------------------

BASIC FIRST AID FOR FISH
Test the water for ammonia, nitrite, nitrate and pH. Recheck the nitrite and nitrate.

Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.

Do a 75% water change and gravel clean the substrate every day for a week or until the problem is identified. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.

Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.

Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.

Post clear pictures and video of the fish so we can check them for diseases.
You can upload videos to YouTube, then copy & paste the link here.