My ceylon puffer is very pale and lethargic this morning after I performed a water change last night. He seemed fine afterwards, I say that because sometimes he becomes stressed when I work in the tank (other times he's curious, even excited, during water changes). He has been swimming around, but not with his usual vigour... he mostly perches on the fake plants or driftwood. His breathing seems a little laboured perhaps, but I'm not sure if I only think it's abnormally so just because I'm worried. His tail is fanned out as usual. He was disinterested by food, which is not normal, even if he's just eaten. Just before making this post I laid an airstone next to him.
pH: 8.0 Ammonia: 0 Nitrite: 0 Nitrates: <10 Salinity: 1.014
Feeding regime: Every other day with combination cockles, mussels, shrimp and blood worms.
Water change schedule: At least 50% per week.
No products added recently (except dechlorinater and salt to new water).
No changes at all except adding additional filtration.
Aquarium has been set up for six months, fully cycled from the start by using the gravel, filter etc from previous tank. Overall I've had him for nearly 2 years and he's always been healthy/seemed happy.
I'm about to do another water change in case there's something polluting the water that test kits don't pick up... I'm usually so careful I can't think what it might be, but a w/c can't hurt.
I know it's silly to be getting so upset, and I should tell myself he's 'just a fish' but he's my little puffer buddy and I hate to think I could lose him.
Here's a picture:

His colours are usually so vivid, it's really a worry. Please help if you can. Thank you.

