Sick Livebearers

[Murphus]

Hi guys, I'm new to keeping tropical fish! I have a 35l aquarium that has been set up for about 6 weeks now. I have two swordtails (they were both females but one now appears to have turned male...) and two dalmatian mollies (female). A few days ago I noticed that one of my dalmatian mollies looked a little chubby, and since I've only had them two weeks I presumed that she must have been pregnant (or carrying sperm) when I bought her.

This afternoon I got a little clown pleco, and also some live plants (I'm not sure of the species but they are long grassy reeds, and I'm sure they just called it 'grass' in the shop). After introducing the pleco and the plants everyone seemed happy, but I've come home three hours later to find all of my fish are suddenly very fat! They also seem to be pooing almost constantly. Are they sick? If so, what should I do? I've removed the plants in case they are in some way to blame for the sudden decline in the health of my little fishes, and have done a 30ish% water change.

Thanks in advance! I'm very worried

Also all the water parameters are fine, and I only feed the fish once a day (flakes).

 

[fluttermoth]

Can you post the actual numbers from your tests, please?

 

[Murphus]

Nitrite and ammonia are both 0, and nitrate is about 5. I am however using strips and I understand now that these can be unreliable, so perhaps my readings are misleading

 

[OldMan47]

Start with a large, maybe 30 litres, water change using your dechlorinator. Don't forget to shut off the filter and heater during the change. Regardless of water test results, a large water change is always a best first step when fish suddenly take a turn for the worse.