Aggressive or schooling swordtail?

[RoseTeeco]

I’m curious if anyone can give me some thoughts on what to do with my swordtail? I’ll attach screen shots below but essentially this little stinker will follow the molly persistently around the tank, and occasionally pick at him. Everything I read says swordtails are supposed to be great community fish and that mollies are excellent tank mates, so what is going on with my swordtail?

More info - this whole debacle started when my kiddo wanted a little 3 gallon circular fish tank like a year ago. We started it all up and populated it with this swordtail and a guppy. Those two were fine for a bunch of months but then at one point the swordtail started doing this same sort of thing where he would follow the guppy all over the little tank; eventually the guppy died. As relatively new fish owners we weren’t entirely sure what was going on so we went and got a danio to replace the dead guppy. That went fine for another couple months and then one day the danio popped up dead. I didn’t see the swordtail harassing the danio, but I did notice the cheap little filter for the aquarium didn’t seem to be working well so I convinced myself that maybe the danio died bc the filter wasn’t doing it’s job, and then went a step further by getting a 20 gallon tank and starting it cycling thinking I would just move the fish to that tank. Once my water tested good I got a guppy and a molly, and then added this same swordtail to the 20 gallon tank. The swordtail loved the freedom of his new tank and did laps like it was his job, but eventually he found the molly and followed him everywhere in the tank. I let it go on for a few days hoping it would settle but it never did, then the guppy turned up dead, and still the swordtail wouldn’t leave the molly alone. Finally (this all happened in less than a week) I just moved the swordtail back into the 3 gallon tank, which I have since termed the “timeout tank.” And maybe related but maybe not, the molly died 2 days later. Still trying to believe the best about this swordtail, I assumed my tank wasn’t cycled right so I obsessively tested my tank water and found there did seem to be a non zero amount of ammonia so I cycled it for a couple more weeks, and then when it was ready we went and got a new guppy and 2 mollies. I was dead set on leaving the swordtail in the timeout tank but he seemed to be decreasingly less and less active over the last couple of days, and he wasn’t eating much, but he would float in his timeout tank staring longingly at the 20 gallon tank. So we relented and decided to give him a second chance. I tested the water in both tanks and made sure it was comparable between the two tanks, and once satisfied I moved the swordtail back into the 20 gal with the guppy and 2 mollies. Things were initially fine, for like 5 minutes, but then the swordtail found the Dalmatian molly and started following him everywhere (pictures below). I let this continue for a few hours but it was nonstop and I was concerned the swordtail would just keep it going until the molly was dead. So now the swordtail is back in his timeout tank and the two mollies plus guppy in the 20 gal are (fingers crossed) happy.

All this to say - I feel really badly for the swordtail, I don’t want him to die from loneliness, but at the same time I can’t let him pester his tank mates to death. Anyone have any other thoughts on what I can try here?

I should also say I tried adding api stress coat when all 4 fish were together but that didn’t seem to change anything. The water quality should be fine, ammonia is close to zero, nitrites are zero, nitrates are zero, ph is a bit high (7.4-7.6ish), but it’s that way in both tanks. Any thoughts or advice anyone has would be greatly appreciated.

[AquariumExpert]

Interesting problem... sorry to hear about it. As far as help goes I have some info that may help and we can figure it out from there. I had a similar thing happen to me with swordtails and mollies. Typically they were fine, but in my case I had more fish. Now all fish obviously are different and have personalities but certain habits and traits spread across a species. You might want to consider getting another 2 or 3 swordtails as this one seems to be lonely but a little harsh when it comes to playing. The mollies most likely died due to stress from the swordtail wanting to play but he was too rough. (Did you notice any scarring, fin tears, or erratic swimming from them before?)So personally I would recommend getting him a playmate more his size (this is common for omnivores like barbs, and etc.) Whether that be another swordtail, or something like a platy, a tetra, or danios. (All of them would need two or three at least of their own kind to keep company). So to sum it up, your swordtail needs a friend who he can really tussle with. Another swordtail or two allow for them to pick on each other and not the slower, peaceful mollies. To state for ease of my mind, I am not an expert on these fish and have only learned through the ones I have kept and friends experiences. This is what I’ve learned and am now passing on to you, it may or , may not work but this is what I simply would do in your case. For others joining or commenting feel free to correct me, or fact check me. I would rather be wrong once than spread false info! Keep us updated and best of luck!

[RoseTeeco]

Thanks for the insight! I had thought about perhaps getting him some swordtail playmates but here are my concerns:

1) I’ve read (anyone with experience jump in here) that male swordtails can be aggressive toward each other. The last thing I want is to get a few more and have them all fight until only one remains. Or worse to kill the other fish if I put them all together in the 20 gal.

2) I’m a little worried the 3 gal isn’t enough space for the one swordtail, would more exacerbate that concern? I think it works somewhat because he can basically swim in infinite circles (bc it’s a cylindrical tank), but not sure if that’s valid.

3) If I found him a few females for company, then I can alleviate concern #1 but then I have to worry about babies, which would crowd the 3 gal quickly. Or, if I put him and his harem in the 20 gal, will the male mollies want to mate with the female swordtails (again, the internet collectively seems to think this could happen).

What a mess.

[WildFish99]

A great solution generally is stocking with either fish that the male cant breed with but have the same amount of play energy (you would want a school of them so 5-6) or getting the male 3-5 females so there are enough of them to take the pressure off eachother. Having lots of hiding spots should help as well.
I had one really agressive male swordtail a few years back in my 30 gallon. He would chase and kill every fish I put in there, sometimes it just doesnt work out well. He got the tank to himself for about 6months before he passed.