Help, sick fish maybe?

[wookie]

I've got a 55 gallon brackish tank. 3 puffers (2 GSP's and one Figure 8) and 5 Monos.

Tuesday night (8 days after my last water change) I scrubbed some algae off the tank walls (postponed water change for another less busy day) and then fed the fish bloodworms and little krill bits and they didn't seem to excited to eat. I didn't notice or think anything of it until the last 36 hours when the problem seemed worse.

Wednesday morning I went to feed them again and they totally ignore the food. Normally the Monos go ballistic at feeding time, darting back and forth and all over the place. It's quite entertaining to watch. This time they just swam back and forth through the food. This food was bloodworms and krill (different source this time). The figure 8 seemed to eat fine, but the GSP's were also a little weird. More black than usual, they didn't seem to brighten up from their sleepy state like usual.

So I panicked a bit and changed 10 gallons/18% (making sure to get out all that food that wasn't eaten from last night or Wednesday morning) also thinking it had been 9 days now since my last water change. Before taking out the water I grabbed a sample to take with me to the pet store for testing.

The pet store tested the water when I went at lunch and said ammonia and nitrates (or nitrites??) were pretty much at zero, said I didn't need to worry about changing my water as much as I have been as the tank seems pretty good. Okay, that's great.... But what's the problem then? Suggested a pro-biotic additive that is good for ick, and some other stuff. A red bottle for fresh and salt water. Added the recommended dose Wednesday when I got home.

Wednesday night same as usual. The 2 GSP's seemed sleepy and were darker than usual, seemed to be doing some weird darting around every now and then. Figure 8 seemed fine. The Monos weren’t as groupie as usual and seemed off.

Fed them a small amount Wednesday night. Same result. Very little interest. Except from Figure 8. I think he's fine. GSP's might have eaten a bit but seemed pretty disinterested.

Thursday morning tried some brine shrimp. Same result, Monos ignored it totally, swam through as if it wasn't there. Figure 8 happily seemed to be eating here and there. The GSP's seemed a little interested, still a bit black on the bellies and darker overall. Still seemed to be in their sleepy state.

I did notice the mono's seemed to be hugging really close to the live rock (one small chunk) almost like they were cuddling it and hiding from the power head current. As well, the GSP's were still doing a random twitch every now and then, but this time I seemed to be able to interpret it as almost rubbing their sides against things. Like plants (fake), the bottom, the filter, the blue bubbly thing, almost as if they had an itch and the twitch was them trying to scratch themselves (the twitch I noticed before but didn’t' decipher). This could also be me imagining things as I'm looking too much into this.

Any suggestions or ideas?

Thanks so much for reading the long story!

EDIT: Just wanted to add, due to the fact the tank was quite clean before I did the water change I'm assuming I'm not overfeeding. As well we've had them for a while (added two monos about two weeks ago) and this is totally new behaviours.