I am new to the hobby and have 3 white cloud minnows in a 28L coldwater tank. The tank was cycled before keeping the fish. I do water tests almost every other day now, and there is no ammonia or Nitrite in the tank but one of the 3 minnows are showing some abnormalities. Please refer to the pictures attached. The belly is bulged with something sticking out of it from behind. This has been the case for the last 7-8 days. To be honest this particular fish had a bloated tummy ever since I bought it, but it has gotten worse now.

I would highly appreciate some help here, as I dont have any experience about the same.
What do you think is there problem here and what can I do to help the fish?

The most likely explanation is that the fish is constipated, as evidenced in the photo by the poo sitting out from its vent but not yet dropped off.
Check out https://www.fishkeeping.co.uk/modules/ ... esheet.php?caresheetID=73 and https://www.seriouslyfish.com/species/tanichthys-albonubes/ for:
* an appropriate diet (eg include peas with shell removed and chopped into tiny morsels plus frozen/live foods - and try to feed little and often rather than one large meal daily) which should help;
* other aspects of care including minimum tank size to ensure that the fish gets appropriate exercise, etc.
If a more varied diet doesn't work after a fortnight or so, or if the situation gets worse in the meantime or other symptoms develop, do let us know - and we'll help further.

Thanks a lot for the reply. I tried feeding one cut-up peas to the fish tank and it seemed that the minnnows didnt want to eat much of it. On the other hand the Zebra Danios feasted on it. Should I isolate the constipated fish and try feeding separately? Also, thinking of offering some frozen Daphnia to the tank in a day or two - would this be ok?
The main question at this time is - does the fish look infected for it to be isolated? or does is it simply constipated?
Thanks

I'll have to be brief (apologies):
* If you have a spare tank and filter, and therefore can isolate the fish, then that wouldn't do any harm - gives it the opportunity to eat and reduces the risk of infection (if it has one - the bulge looks somewhat distended but actual cause unclear at present - time may tell).
* Frozen daphnia = excellent idea.
* If you have zebra danios in the 28L tank too, then, for their and WCMMs' health, definitely an upgrade of tank size necessary soon - cf https://www.fishkeeping.co.uk/modules/ ... esheet.php?caresheetID=20 and https://www.seriouslyfish.com/species/danio-rerio/
HTH.

Unfortunately, using peas, Daphinia and a much varied diet hasnt helped the case for my WCM. It doesnt show any other symtomps of being in stress or sick, its eats and is active in general. Any ideas on what I should be doing next?

What are your test results for ammonia, nitrite and nitrates (and with what brand and type of test kit)?

Ammonia = 0 ppm
Nitrite = 0 ppm
Nitrate = 10 ppm
pH = 8.0

Ammonia, Nitrite, pH from the API master kit and Nitrate from Salifert Nitrate Test Kit.

You could try adding Epsom salt to the tank at 1 tablespoon of Espom salt per 5 gallons. Keep it at that level for a couple of weeks by redosing if you do any water changes. Note: Don't do this if you also have soft water fish in the tank, instead treat the sick fish in a separate tank.