How big can copepods grow to?

The friendliest place on the web for anyone with an interest in aquariums or fish keeping!
If you have answers, please help by responding to the unanswered posts.


Aquarium Advice Freak
Jan 27, 2004
London UK
My copepods population have been growing steadily since I introduced LR over 2 months ago. Recently I have notice some big copepod like crustacea about 8 to 10 mm long. It is more like amphipods, brownish, flattened from side to side and with tail that cures up.

The question is what size do they grow to and whether they are harmful to the fish at that size?

Do I need to control the copepod population? I don't have enough LR to support a Mandarin, will a neon goby snap up the copepods?
I don't think they get much bigger than a quarter inch, although I saw a picture of a deep water isopod that was huge, probably 6-8". I have never noticed a problem with copopods or amphipods ;)
Okay. Tonight I used my touch to search for more and found hundreds of them. They are pretty horrible looking and pretty big, definitely amphipod like. So they are pests right? Will their population stabilize or continues to grow?

Also found a couple of bristleworms. Is it okay to leave them or will they breed like rabbits?
I would leave them all.. I havent ever heard of them doing anything bad..

I leave all mine..

I leave all mine in as well. Most of these little critter will help to clean up the tank and also provide food source for some fish.
I have 55 lbs of LR that I have bought as recent as a couple of weeks ago to back to Dec of last year. I do not have ANY little critters that I know of. I have gone in late at night and turned the light on really fast and see nothing. Is this normal not to have any of these? I have bought all of my LR from locals so I assume if I have any at all, they are slow to reproduce....I have a 125 SW and only 5 small fish.
I have recently ordered LR from so Im hoping to get some of these. I had heard it takes 4-6 months to possibly see any of these little this true as well?
Those photos are awesome. Never thought of it getting that big, I hope not in mine anyway.

Fishman, if you look closely on your tank glass, you might see some of these copepods like lice running across the surface or free swimming in the water. The bigger ones you might find by shinning a torch into the holes and cracks of your LR. If you don't see any then you can introduce them by buying more LR. The new LR should seed your tank. When I bought my first LR, it took one week to get into a copepod bloom. After about 4 weeks, the amphipods (I think) have grow to about 8mm.
Top Bottom