Pea Puffer Help, poorly fish :(

[Kerrynic]

Hi Everyone, I could do with a bit of advice for my pea puffer, I originally had two but now have the one and it seemed happy enough until this last week I noticed it was starting to get skinny. I went on holiday Friday and did a tank clean and fed the little guy which I thought looked a bit skinny but he was swimming around and acting pretty normal and usually comes straight to the front of the tank when I enter the room which is sweet. I have just returned from holiday to find it lethargic (thought it was dead initially), hardly moving, not wanting to eat (frozen bloodworms). The tank seems fine, nitrite 0, no ammonia showing, no chlorine. Checked the filter which is working and water is around 26degrees right now. Tank has a live plant, marimo balls, stone gravel, and an ornament to hide in which I have removed so I can keep an eye on the fish but now it is hiding in the marimo balls. I do have some anti internal bacteria treatment (Interpet) but not used any yet.