The growth rate is hard to judge for any fish, because it depends on so many factors, but 2.5-4 cm/1-1.5" per year is probably achievable for younger
or
(they are pretty similar, if they aren't the same species).
Judging the age of fishes when they are bought is almost impossible - We can have a guess that a 4cm fish is probably about a year old, a 8-10cm fish is two years old, and a 12-15cm fish is probably 3-4 years old. But that is a completely unsubstantiated guess on my part - and more importantly, if they fish is nearly fully grown, there is no way to tell how old it is, as once fish reach a certain size, they spend less time eating and more time trying to mate, and they are genetically programmed to not grow much more at that stage either, but fish DO grow their entire life. #
If the fish are known to be from the same brood, then the bigger ones are PROBABLY male. But that's a pretty poor judging factor in a shop, as you have no idea if they were even collected in the same location [probably], and you may end up buying some from the brood that was born a few weeks earlier, and some from the later brood - in which case who knows what happens. You have to rely on statistics to get it right here.
If you don't mind me asking, how much are you paying for these? I've seen some tiny L128 that were completely silly price compared to big ones - I've seen large L200 and L128 in MA branches at around £70 each, and they are definitely sexable - it is obviously a better choice to buy three of these than 8 unknown sex ones at just under 30 quid each and not know what you get - you COULD end up with 8 males - and you will have spent some £50 or so feeding them for the two years until they are sexable. And Pier Aquatics are usually a lot less expensive - and if you want a group of these fish, it's best to buy them all from one shipment, so you don't end up with a mix of fish from different locations.
Haavard wrote a thread on breeding L200 some time ago (actual title, I think is "Breeding Hemiancistrus subviridis").
Oh, and I moved this topic to the "correct" forum.
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Mats