Worried about my potentially pregnant molly!

I’m not sure but I think my dalmation Molly is pregnant but I don’t know how close she is to giving birth. My male molly has always been agressive towards her but has been chasing her relentlessly the past few days. She’s been hiding and really inactive. I’m not sure if she’s sedentary because she’s close to birth or she’s just hiding from the male. My tank has a lot of moss for her to hide behind but he always seems to find her. I don’t think he’s trying to breed, he seems more interested in fighting her. She used to be a lot more active before today. She has also been picking at the moss and eating peas but seems to have a slightly decreased appetite. My tank is a temporary 5 gallon that they’ve been in for 3 weeks with 5 guppies and the 2 mollies total (my 25 gallon got a crack in it so I moved them into the smaller tank for now until my new tank ships in). Since it is a smaller tank I’ve been doing partial water changes every day and using reverse osmosis filtered water, parameters seem good. Plants are from a local river and all the fish have been picking at it and eating it, seem to really like it so I don’t think it’s an internal problem, but I did lose 2 female guppies a week ago out of the blue so I’m a bit worried. I’m not sure if I should isolate her for a while if that would stress her out even more? Any ideas if this is pre-birthing behaviour or she’s stressed out?

Mollies (and guppies) need hard water which contains lots of minerals. RO strips all the minerals out of the water. Are you re-mineralising the water? Unless you have a particular reason not to use tap water you should, or add minerals back into the RO water.

In addition to what Seangee said about needing to add minerals to the reverse osmosis (R/O) water, mollies need water with a general hardness (GH) of at least 250ppm, and guppies need a GH of at least 200ppm.

Your female molly is being bullied by the male. If you have a breeding net you could put the male molly in that and it would reduce the stress on the female. Adding more plants would also help.

Livebearers are best kept in separate sex tanks, 1 tank for males and 1 for females. Then the females don't get harassed continuously. If you can't have separate sex tanks then try to keep at least 4 females per male and preferably 6 or more females per male.

seangee said:

Mollies (and guppies) need hard water which contains lots of minerals. RO strips all the minerals out of the water. Are you re-mineralising the water? Unless you have a particular reason not to use tap water you should, or add minerals back into the RO water.

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Yes I should’ve mentioned I do supplement the water with minerals along with the water changes. I used to use conditioned tap water but it would keep killing my fish.

Kimnar said:

Yes I should’ve mentioned I do supplement the water with minerals along with the water changes. I used to use conditioned tap water but it would keep killing my fish.

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Then Colin is most likely right. I didn't mention the male / female ratio because while they are in the 5G you obiously can't add more fish. If you can separate them do, if not chase up the new tank