Oscar eating rocks and spitting out gills

I recently moved house and my ~10 or so year old Oscar has decided to stop eating for a week now and chewing his rocks and swallowing them and spitting them out of his gills - it looks awful but mainly appears to be doing it when sitting near him, he has been swimming around and carrying on as usual but not interested in his food anymore which he was very fond of before moving. We did transfer him into his old slightly smaller 3ft tank for ease of moving but apart from that I was able to use majority of his water and transfer quite easily as though when I got his larger tank a few months ago which went fine. Any ideas??? I’ve read they can go a month without food but the spitting rocks out of his gills is worrying. Thinking he may be unhappy in his old tank but wanted him to settle from the move before then transferring him again, but may have to consider doing this sooner. Thanks!

Edit:
Photo taken during move, he is usually in a lovely spacious 4ft tank with large rocks and the occasional plant before he destroys them to pieces. I see his environment does look rather miserable from this picture but only recent close up

 

I'm not sure what the purpose is but many fish species both in the aquarium and in the wild do that rock eating behavior all the time and it doesnt ever seem to affect the fish negatively. As for the not eating it could be that the oscar is simply sick of the food you've been giving it so far. They are very intelligent and thus require more than just the same thing day after day. Try feeding it a more varied diet. Also, how big is that tank. It looks too small for an adult oscar in the picture.

Edit: Many cichlid species will deliberately swallow substrate and spit it back out in the hopes of sifting out any edible organic matter from it. The diet you are currently giving it could be the reason for this.

 

Absolutely normal cichlid behaviour, many dig in sand and a lot chew sand sifting it for food particles. In this case it seems though as if this is the only way of interacting with its environment it has.
I would probably replace the (much to rough) gravel with sand.

And to be honest: That fish deserves a tank 3-4 times the size of the one it is in right now. Sand substrate, some driftwood, maybe some rocks. This puddle it sits in is not appropriate for a fish of that size.

 

I should have mentioned the picture is mid moving as I was draining the water out - he is in a spacious 4 ft tank with large rocks he has a love hate relationship with. The main concern is he hasn’t previously ingested large amount of rocks and actually swallowed them to spit them out his gills - just moving rocks around and spitting them out normally yes but not out of the gills which I’m worried may hurt him if it continues. I may give it a little more time and hope for the best

Absolutely normal cichlid behaviour, many dig in sand and a lot chew sand sifting it for food particles. In this case it seems though as if this is the only way of interacting with its environment it has.
I would probably replace the (much to rough) gravel with sand.

And to be honest: That fish deserves a tank 3-4 times the size of the one it is in right now. Sand substrate, some driftwood, maybe some rocks. This puddle it sits in is not appropriate for a fish of that size.

 

How long ago did you move, and I am not clear from your posts whether he is still in the 3ft tank or now in a 4ft (still pretty small for an Oscar)?

As MacZ says, moving rocks around, digging and sifting the substrate is normal for cichlids, and I agree that sand that he can naturally expel from his gills with no harm would be much better for him that coarse gravel. Eating large rocks though is not really normal behavior though, and neither is refusing food. These are "intelligent" fish that in the wild would perform a complex set of behaviors including foraging for food, courtship, spawning and parental care behavior and territorial defense. There is little opportunity for this type of behavior in a small glass box with no tank mates and just a few rocks. Abnormal behavior in captivity is seen in many animal species and is a big issue in zoos and animal rescue centres (bored parrots pluck out their own feathers until they are bald, monkeys bite their tails and create deep wounds that become infected, bears pace up and down and perform other abnormal stereotypical behaviours). The solutions to these problems involve environmental enrichment, which can take many forms. In your case adding some pieces of driftwood that extend to the surface would give more structure and new spaces for him to explore and investigate. A few large Indian almond leaves that he can move around and forage underneath would be good too, especially if you can hide tempting food underneath them. You don't say what you have been feeding him with, but providing variety and including some live food might encourage him to eat.

Of course, at 10 years old he is getting on, but I doubt if this behavior change is just due to age, as they can live to 15 or even 20 years.