I have a tank with 3 guppies, 2 fire belly newts, and one Borneo sucker.
Guppies, being guppies, have young frequently. I would like to control their population, if possible, by introducing a fish that will eat baby guppies, but not bother the other inhabitants.
This is a cold-water tank.
What is the best fish to introduce that will "naturally" control the guppy population?
Firstly, I don't know about the sucker, but the guppies are not cold water fish. You really need to get a heater and keep the water in the low-mid 70s °F (21 to 24 °C). I wouldn't think the newts or the sucker would like it either, but I won't say different, since I don't know for sure.
As far a controlling babies, you have a couple of options. One is to remove the females. This is easiest, since the males are the pretty ones. Pick out the males you like at the pet store and take your females back to them. Any pet store will usually take unwanted fish if they look healthy. They may want a sample of your water, but I've never had one turn me down. They just resell them. You can do this with your babies as well. If you have them, you can leave them in the tank with plenty of cover. The parents will eat the ones they can catch, usually the weak and deformed, which saves you from culling. You can let the other grow till you can tell their colors and keep the ones you want, taking the others to the pet store.
If you really want something to eat them, I'd suggest African dwarf frogs or crayfish. Either one are cool to keep and predate on things when they can. The crayfish have the added benefit of being a tank janitor and their shells change colors gradually with each molt to match the gravel color as best they can. I once had one go from a jet black 1/4" (0.63 cm) baby, to a 3.5" (9 cm) long blue/white behemoth on white gravel over a couple years. Then he got big enough to start killing the healthy fish and had to go. You can catch them in creeks easily, but just get one and even in a 35 gallon (130 L) tank, I had two babies fight to the death. They're territorial. Good luck.
Guppies, being guppies, would control the population themselves if it's not a heavily planted tank and doesn't have alot of hiding spots. Guppy-nabilism. Delicious.
