Tiger Oscar stopped eating :(

[firstchoice]

Hi

About 6m ago I accidentally inherited a Tiger Oscar.
It's in a Juwel Trigon corner tank (fish is about 5" atm)
He fed happily on defrosted prawn (as last owner specified) until about 3 weeks ago when he stopped eating and is now listless and his dorsal fin has gone raggy.
There is also an upside down shark in there who has stopped eating too.
The tank is kept clean and about 24 degrees but the light unit has bust and atm I cannot afford to replace it (c£90).

Any advice would be welcome.
I feel if he is not happy in his environment it would be kindest to rehome but I am worried to do so now might finish him off?
I hope there is something I can do to help him as clearly something isn't right as shark not eating either?

[EauRouge]

Have you got a water test kit? When's the last time you did a water change? I'm guessing you know already how big he's going to get. They are beautiful fish. What's the upside down shark, some kind of synodontis?

[firstchoice]

Water change weekly. 2/3 tank. Filter sponges washed out in rotation.

Yy realise he will be huge and will need to rehome as I cant get n 8 foot tank but don't want to stress him by rehoming whilst not eating / poorly.

Yy, the shark is a synodontis type.
He hides at the back under the bogwood but creeps forward for prawns.

Do you think the light unit not working could be the issue?
The tank is in a corner of a well lit room.

[EauRouge]

Could be the light if the tank is planted, dying plants would produce ammonia. Have you done a water test?

[firstchoice]

sorry, it's taken me aaaages to reply.

Water test results all clear, so its not that.

Poor soul is just lying there now, though