My smallest clown loach over the past few day's has been acting really wierd. It usually swim's around the tank with my other loach, but it's started being really aggressive towards the other loach. They have been together for 5/6 years and there has never ever been a problem.
It's really protective of an ornament all of a sudden (that has been in the tank for ages) and i often see it carrying pebble's and bits of plant inside the ornament.
I don't want to move the ornament and disturb what he's been doing because he's put so much effort onto it.

I have no idea if it's male or female and i've been told it's near impossible to tell.
Is there any reason for it to act like this?

It could be that you simply have a territorial fish, however without knowing more about the tank I can't really offer much advice.

If the problem persists, rearranging the tank dramatically may be a possibility. The social dynamics of unsettling fish can often work to the point where all aggressive and territorial behaviour ceases to exist amongst the tank's inhabitants.

However, ultimately the best way to dissipate territorial behaviour is to simply buy more fish. Where Clown Loaches are concerned though, this is something I wouldn't advise unless you have a tank of 6 x 2 x 2ft or larger.

But what i don't get is why it's only started doing it this week. Why now after 5 years or so.
Do fish go through like teenage stages if you know what i mean?

you mite have yourself a female who has laid eggs and is defending them!!! i really dont know so if im wrong im sorry but, if you have, id say if you want them to hatch put in alot of plastic plants (so your water doesnt go green) and be carefull when doing water changes incase you pick up the babies if it is this then you should know in about 2 month as you will see them swimming around if the others dont eat them. i had fish and one was really aggresive to the others, but they only took so much and then one turned on it, and the aggresive fish was put in its place, it didnt attack again!!! so its up to you what you deside to do but, i hope its babies i love baby anything soooooo cute lol.

clown loaches eat there own eggs unless taken away don't they?
From what i've read in books and online, they don't generally breed in captivity. But i'm not sure.
I love babies things too!!

I can't really visualize how artificial plants are going to prevent water from turning into a pea-soup...

i dont have a clue it was just an idea that popped in my head of what it could be hope the fish calms down a bit though

its just that i noticed when i had my tank a while ago now that the live plants i had in turned the water green but when i got plastic plants it stayed clear

i still did all the water changes and everything i was suppose to do but i think i had to many plants in tbh

Floating plasic plants to shade the water

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i did think of that at first, maybe it's just going through a rebellious and stroppy stage.