I need some ideas on how to induce natural selection to my guppy tank. I have 35 guppy fry and I need to kill some off so only the strongest and most intelligent survive.
I would use a predator but it doesn't seem like a good idea for fear of killing all my fish.
Something like a goldfish maybe ? To just eat a couple of the fry but maybe not always eat them
No, bad idea. Goldfish are poop machines, and they grow huge. They really are more a pond fish than anything else.
I would instead go to the store and buy the betta with the largest/heaviest fins you can find. That way only the slowest/most stupid fry will get eaten.
I would also get some java moss, tie it to a rock or whatnot, and with every water change gently comb it out with a smooth wide toothed comb. That way the individual strands become long and bushy so there is a shit ton of hiding places within it for the fry.
I would also get a broad leafed fairly tallish plant for your betta to rest on when near the surface. Lugging around those massive fins takes a lot out of a fish.
Hell no, if you do get a betta, you want that fucker fat and lazy. Otherwise he will shred all of the adult fish in your tank. What you want is a betta that will pick off the occasional guppy fry that stupidly wanders too far away from it's hiding place. Not a betta that is a murder machine.
And as for the other suggestion, that's a bad idea, if you feed less, the adult fish will become sickly due to starvation, and then you'll simply wide up with a bunch of fish corpses in your tank, which will cause an ammonia spike that kills ALL of your fish.
As it is, guppies are pretty damn stupid, the parents will probably end up cannibalizing all of their own children anyways.
Add a couple of Ghost Shrimp to the tank. They help a little with cleaning, and eat fry who are too slow to start swimming after birth.
That doesn't really help with your current overpopulation however. I'm no expert, but all the 'predator' fish I can think of are over eaters, and then you'd have no guppies.
Ich is naturally in every fish tank. It'd be nearly impossible to have a tank that doesn't have Ich in it somewhere, lying dormant. All it takes is a lot of stress to bring it out of fish. Often it's when they first move to a new enviroment, like transporting from the dealers to the LFS, or from the LFS to you. But it can happen just as easily if enough stress is caused.
Get some of those little fresh water crabs and put up a rock for them to get out of the water on.
They don't get big enough to eat adult guppies and will hunt for smaller fish if they can't find any dead meat laying around.
Do not go for a crayfish though, those things get huge and violent towards larger fish and tend to ignore the fry.
you can't induce natural selection in a fish tank, dude, it's too small. the best you can do is have a huge fish tank, plant it heavily, and introduce a few smaller gourami or betta. Some betta will be aggressive to adult guppies, so keep that in mind.