This was initially asked as a comment on the bristlenose plec page however I think its a pretty good question to warrant a more indepth answer.
Hi,l have an established 250 later fish tank,with 18 gold fish,and four bristle nosed pleco’s,l have plenty of clay pots ect,very clean tank,and a regular tempature of 24 C,my fish are a happy bunch,my pleco’s are reaching 6 months old,will they breed,and when ?,do l need to add more artificial plants to protect any Fry,am l a good keeper of fish,or am l doing something wrong.
First things first well done a nice clean tank with a stable temperature (and I assume water parameters) is excellent fish keeping regardless of fish kept.
Now for your questions. When will they breed? well first up you have to make sure you have at least one male and one female difference is easy to spot male will have spikes around his head even if they’re not very big they should be showing by 6 months, he’ll also be a little pointier and thinner around the head than the female the female will have no bristles a stubbier almost 50p shaped head quite wide around the gills, she’ll also be a little tubbier into the belly area too.
Now if you have a pair 6 months will be starting to get ready I’ve had some do it much younger (younger than I thought was actually possible!) some have been much older before they’ve spawned. Just be patient and look after your tank and they’ll do it themselves.
Its worth noting though that while the fish in general may be nice and happy around breeding time horny males when you have more males than females may get aggressive with each other and so stop each other breeding. Its also worth noting that first time breeders don’t always get it right sometimes they eat the eggs, sometimes the kick the eggs out, sometimes they forget to stay in the cave and look after them. Its nothing to worry about it just happens they usually get the hang of it after a couple of attempts.
Artificial or any plants for that matter are optional with most plecs alot of breeders just breed them in empty tanks with just caves in. You may want to check your breeding pots have only one entrance and not 2 or more remember the male needs to trap the female inside the cave/pot. Also make sure the entrances to the caves are quite small with most plecs they’ll actually fit in caves that you’d think were far too small for them they usually prefer it a little tight. One of my biggest bristlenose males always preferred a cave that was no bigger than 2x2cm on its entrance i was always shocked he fit let alone him + a female.
With your males look for a bright red patch on his head (harder to stop on common bn’s but still visible) when this is proper glowing red it means he’s in his prime looking for a female. You may also notice that he picks a certain cave and stays in it / hangs around and on top of it and rarely moves that’ll be the cave he’s now picked as his breeding cave and its just a case of him encouraging a female into it. He’ll become very protective of that cave.
All in though with BN’s just leave them too it, keep the water in tip top condition and feed them well with veg etc and when they’re ready assuming you have males and females they’ll eventually get jiggy with it with very little input from you. Always best when you’re first starting out before you start with the tricks to encourage them to spawn faster / more frequently.
As usual with this hobby patience is key!
Good luck I’d love to see some pictures of the little wrigglers when they do breed.