Paired Sleeper Gobies arent getting along anymore...

[Stray32]

I have paired Sleeper Gobies that have been getting along for months. They sift sand together, hang out in their burrow together, etc, etc. I noticed in the last few days that only one of them is out swimming around, while the other stays in a separate burrow. At feeding time, the swimming one seems to prevent the other from coming out to eat. I had to target feed it today whilst staving off the other.

Have they bred? Why would they have suddenly turned on each other? The one in the burrow is quite thin and doesnt look nearly as healthy as the other.

And yes, I am sure they were actually paired when I bought them, it was obvious.

 

[Mattchamp]

It may be territorial aggression I don't think they would do that to each other if they were mating