Buying Fish Already Pregnant?

Hi Claire,

Welcome to the forum!

A couple of things....firstly, you say you set your tank up around 4 weeks ago. Did you cycle it, or are you doing a fish-in cycle? If you didn't and you haven't been doing daily water changes, then that could be why one of your fishies is looking out of sorts. You have to make sure that all the ammonia produced in your tank, like the waste your fish will produce, which will then produce ammonia and also all nitrites that will be produced from that, are cleared by the correct bacteria in your filter. There are a couple of excellent threads you should read through written by Sue, on both fishless and fish-in cycling. If your tank isn't cycled, your fish will be poisoned by the ammonia and nitrites. This is why you need to get a test kit and test your levels daily....and do water changes immediately you spot either of these.
Do read the cycling techniques though, they will walk you through very easily what needs to be done.

As for buying fish that are already pregnant, if you buy livebearers like Mollies, you will almost certainly be getting pregnant fish. These types of fish are prolific breeders and even if still very young will almost definitely be pregnant if they have spent ANY time at all with a male They can store sperm for a couple of months too, so even if they weren't in with males at the shop you bought them from, it doesn't mean you are safe from the flapping of tiny fins
My platies have produced from the day I got them almost.....and now their babies are producing young of their own!
After my initial excitement at this and worrying that they would all be eaten by everyone else, I am now quite blase about them, accepting that I cannot save them all....and indeed wouldn't want to as I would be seriously overstocked in my tank otherwise.
I have been lucky that one of my LFS has been willing to take the babies at any size and number, but I know that this is not usual. Of three I asked, one wouldn't take any, and one would take them after they reached the size he could sell them at....by which time of course, all the females would be pregnant again and so it would go on
I am whittling mine down so that I won't have any more pregnancy issues with them. I will probably just keep a couple of males, but that will mean giving away my original girl, and I feel a bit mean about that.

Anyway, good luck with it all, I think you've got some puzzling to do.
You will get loads of advice from here though, so don't worry you are on your own...everyone is really helpful.

Resa