I have four black mollies and three of which have developed huge bellies and their bum is now a white spot Are they pregnant The 4th is skinnier and sniffs the butts of the other three constantly?

it depends on how old they are if they are 1 year old than yes and if they are months old then no.

*Sorry not sure how I reply to your comment*

Thanks for your answer. They are only about 6 months old. Three of them have just bloated up over the last week or so. I have a golden molly who is now a little confrontational to the three and the fourth skinnier black molly now goes from one fatty to the other sniffing (or putting his mouth to) their backside one after the other and doesn't want any other fish too close to them (doesnt approach any other fish, just these three).

Forgot to mention that there is also a fifth black Molly and they are the tiny runt of the litter 6 months ago and is blissfully unaware of everything!

Regards

Emma