Sudden aggression

[Anemoneee]

I have five fish living together for almost two years and tonight I suddenly found the smallest fish got beat up very severely. Back tail is 80% gone and damage to top & side fins and scale is very badly damaged. Can't seem to swim straight, tail unable to move. He tried to swim but only manager to have head facing downwards.
Now I put him in the breeding net.

My question is anyone have experience with fish got beat up so severely and recover from it?

Why the sudden aggression after two years living together?

Tank mates
Neon Dottyback
Orange Clownfish
Azure Demsal
Panda Banggai
Royal Gramma

Thanks all, any feedback is appreciated.

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[Michael Hoaster]

A couple of factors come to mind. Most fish are juveniles when we get them. Having fish for two years likely means some of them are reaching maturity, which generally means they get meaner, in the confines of an aquarium. Usually this manifests as squabbles over territory, especially if the tank is on the small side for the fish you have. Also, fish that are similar, like your Neon Dottyback and Royal Gramma, see each other as competitors for food and territory. Which brings me to another possible cause - hunger. The less you feed, the more competitive your fish have to be to get enough to eat.

Without knowing your tank's specifics, I'd suggest re-aquascaping so that there are plenty of hidey holes and gaps between rock formations to help separate territories. Also feed your fish more, so everyone gets plenty to eat. You may want to consider regular additions of pods, so they can hunt naturally.

 

[Sk8r]

When a fish reaches a size where a dead end in the rockwork looks like being cornered, he'll kill if equipped to do so. When a fish reaches sexual maturity a pair may form, and start attacking other fish. Always stock for the ADULT size of the fish you keep. You can find this adult size either by looking up the fish's Latin name in Wiki, or by consulting Live Aquaria's handy chart. If the chart says 2' ---they're not kidding. Most fish that aren't minnow-shaped are sold as babies---eg, no HINT what they will be. It's on the buyer to look up that adult size. A baby fish can reach significant size in a year, year and a half.

 

[Timfish]

What they said. Additionally, IME this can happen YEARS after fish are added. An aquarist always has to be ready to rehome somebody.

 

[Anemoneee]

Too bad I didn't see it sooner, and relocate him. But didn't see it coming at all since they been living together for so long. sadly the fish didn't make it the night. RIP.

Thanks all

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