When to euthanise?

My Betta has been lethargic, lacking in colour, struggling to swim and with laboured breathing since Saturday afternoon.
He was extremely constipated until he passed a huge globe of a poo followed by an inch of very dark poo yesterday afternoon. I thought he would perk up, but no.
I am worried that he has suffered from a malachite Green dose I gave him on Saturday - there were white spots on my shrimps’ legs so I thought I’d start the dosing process. I’ve now diluted it all out.
He also might have gorged on a prawn chunk that I had put in for my shrimp, also on Saturday.
I can’t bear to see him suffer. His swimming is slightly lopsided and it’s as if he’s lost power in his thrust. When he goes to the surface for a gulp of air, the last burst of energy he has to exert is horrible to watch.
His water is pristine - he’s had 2 x 50% water changes on Sunday and today. Nitrite and ammonia are 0.
Is it time to euthanise him? At the moment he’s letting himself be sucked against the filter

Attached are 2 profile photos taken 2 weeks apart to show how bloated he is. Please advise

He doesn't look fatally ill to me. He is a strange one physically, but that was before and after.

I don't see the fish as you do, but if he got a large chunk of food, it'll take time to go through. Malachite green shouldn't cause any of this, even in an overdose. If you had overdosed the tank, the shrimp would be dead too.

I would back off, leave him in peace and see how it all plays out.Medicating unnecessarily is a stress, and now you know to chop up the prawn carefully. Because you say you are concerned with how he looks - I think by breeding he is one rough looking specimen of a Betta. But it doesn't mean he's ill. he has a certain character to him.

That’s good to hear about the malachite green. And the shrimp are thriving. Thank you for posting

Absolutely agree, keep his water clean and enjoy that unusual (and pretty) fish.

Just curious.. how old is your Betta?

I don’t know. He was big!
He died last night

Sorry for your loss. It may have been close to his full life.. bettas only live for 3- 5 years . They are often about a year old when you buy them from a pet shop. How long have you had him?

Will you be getting another betta?

2 weeks. But the shop had had him for a while.
I think the big chunk of prawn I put in for the shrimp contributed. I thought that he would ignore it as it was way too big for his mouth, but I found him throwing it around and fully enjoying it.

I would like another Betta. I’ve bred them successfully and I love their personalities. But I’m a bit wary now.
I have a fully cycled 34L tank with 6 cherry shrimp in and now a quarantine tank set up for whatever I get to join the shrimp.
I do like bottom feeders. Plecos are lovely. But bettas are fun.

If you have a test kit, you might test ammonia, nitrate and nitrite. You might do a large water change with dechlorinated water.

If you wait too long to get another fish, your filter’s beneficial bacteria may die off.

That is, assuming your tank is healthy.

Tank is healthy (I think) - water parameters are great (0 nitrite, 0 ammonia) and it’s cycled. I have 2 lots of media in there so I can take one out for a quarantine tank whenever I need to. I’ve just revamped it after getting fed up with hitchhikers on live plants, so took the opportunity to flubenvet and fenbendazole it all (with no fish in), then hydrogen peroxide it, then empty it and dry it. It’s now purring away again with the shrimp in, and I’m continuing the malachite Green treatment that I started on Saturday morning because my shrimp had little white dots on their legs. Shrimp are thriving.
So I’m hopeful that the tank is good! I miss the look of plants, though - had some good advice on here about the ones grown in jelly which come hitchhiker free, so I’ll look into those.

Good advice re beneficial bacteria dying off. I have 6 shrimp in there and feed them shrimp pellets - I’m hoping that will keep things going.

I'm sorry he died.
But now I'll say it - he looked like an almost deformed Betta. The inbreeding that makes all of those bizarre colour forms has a price. I would suggest taking a step back and looking for one with moderate fin growth and a sleeker body.

A well bred betta in good conditions should live 4 to 7 years, but the first 6 to 8 weeks after shipping are the true danger zone. The fish are transitioning from being raised in antibiotics (arriving with gut flora issues) and are vulnerable. It's rarely a matter of any infectious diseases, and more often a question of recovering from the intensive farming techniques. They are usually about 8-9 months old when they are sent out for sale. Some are slightly younger.

I kept and bred a lot of Bettas in the past, but have no intention of keeping any more because the fancy breeding has become so over the top. I'm not into that, but have no disagreement with those who are - it's just what we want to see.