Well ive had a 10 gallon running for a long time and bought some fish yesterday. 3 female, Sword tails and 1 male guppy. I bought the male guppy because i thought they would not mate. But doing some research on the internet it looks like it is possible, is this true? Can sword tails and guppys cross breed?

I used to have these types of fish as my first ones years ago when I was just starting out, and I can assure you that it is quite unlikely that they will cross breed...

Another weird thing about swordtails is that females can change sex if they are no males around, I've seen it happen! but the even stranger thing is, is that they still can't mate, I think its due to a sterility problem or something but I never delved too deep into the topic a google search might yield more info!

Swordtails can cross breed with mollys and platies though as I've had this happen...

But you should be alright with your current stock!

Happy fish keeping

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195G Pleco - 13" Clown Knife - 14" Silver tinfoil barb - 7", 7 Clown loaches - 4-5" 1 Black ghost knife - 7" 2 Tilapias - 4" 2 GT's - 3"

Hello dto...

You're dealing with "Livebearers", all have similar reproductive organs, so a hybrid may be a possibility. However, if I'm not mistaken, the fry are sterile. Don't quote me on this, it's been some time since I've read anything on the subject.

If by chance, your fish reproduce, your 10 G could get crowded pretty quickly, creating a water quality problem. Another, more pressing issue to consider is the possibilty one or all of your Swordtails is pregnant.

B

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Low Tech Water Building Keeper

No, swordtails and guppies cannot interbreed. No problems there.

platies and guppies don't cross-breed either, at least from what I've read they don't.

They don't interbreed - there may have been some things done in labs under conditions we'll never see, but each fish is from a different Genus, and they aren't that close in evolutionary terms. Swords are Xiphophorus helleri, and they'll cross with other Xiphophorus. Almost every swordtail, platy and variatus in a pet store is a hybrid already, as that's partly how all the color and shape varieties were developed by breeders.
There are three wild guppy species identified so far, and they'll cross easily.
As well, all species of mollies will cross in aquarium conditions.