Help! New F8 pacing for hours and not eating

[piffpuffer]

I just got one F8 puffer as my first puffer, still juvenile, around 2cm in size.

I've just gotten a 10 gallon for him so the water hasn't been cycled yet, since it's new. There's aragonite sand as substrate, a heater that maintains the temperature at 24-26C and I've done water conditioning and added bacteria to the filter and left it overnight. Started the tank with Microbe-Lift Xtreme Water Conditioner and Microbe-Lift Nite-Out II Starter bacteria.
Hasn't been cycled yet, so the water test isn't surprising:
Ammonia - 0
Nitrite - 2
Nitrate - 40
Used the Aquarium Lab Multi-Test Kit.
I haven't started adding marine salt yet so SG is still at 1.000 (plan to raise it to 1.005). Will drip-acclimatise him when water has been cycled.
I'm planning to start the fishless cycle today.

So I have the F8 in a 'quarantine' tank. I brought him back early from the fish shop just because the conditions there were horrid and they weren't feeding them well, all the puffers had no bellies he seemed the most active so I decided to bring him back last night to get some food in him. The tank is about a little below half the 10 gal tank, and I used the tank water from the store (freshwater). There's a small sponge filter and he paced around for about 2 hours before finally calming down, I also put up black sheets against two sides of the tank and it seemed to help a bit. I know it's really small and I don't like it, I didn't have much options and the store tank was smaller. I had this tank empty intended for keeping shrimp so I used it. I gave him three small snails and he hasn't touched them yet. I tried again this morning with thawed bloodworms, didn't eat them, he's been pacing and darting around all morning. He stays still at the bottom as a 'break' from pacing and darting then goes back to it. He had normal poop last night but today morning I found a longer stringy translucent one. I read that it may be because he hasn't eaten.

I tried a decoration plant and cave to curb his boredom but it seemed too large for the tank (bought it for the 10g), and he had trouble navigating around so I removed them. The water tests for this tank:
Ammonia - 0.2
Nitrite - 0
Nitrate - 5
Since he isn't eating the worms and the ammonia is 0.2, I'm gonna do a water change today, dechlorinated with Aqumedi Basic Chlorine Free given by my local fish store, and conditioned with Microbe-Lift Xtreme Water Conditioner and Microbe-Lift Nite-Out II Starter bacteria. Going to get some smaller caves for him now. I know he'll be happier in the 10g with all the decorations but I can't move him over until it's been cycled properly. Based in Singapore, so Prime is dechloro is getting shipped over, don't have it yet, will switch to it when it arrives!

Help! What can I do in the meantime? I really want to keep him alive!! I appreciate any help and advice!!

[Pufferpunk]

How is he doing now? Nitrite of ANY amount is extremely toxic.

[piffpuffer]

He's eating now, seems to not like brine shrimp though, he'll eat a little bit of it if I'm not dropping in anything else. He's eating snails (feeding snails algae wafers and spinach), freeze dried shrimp, and thawed bloodworms. He's very active, doesn't glass surf as often, not as much darting unless startled during water changes. Daily water changes (between 30% to 50% depending on how much debris) after feeding, I also changed his sponge filter to one that hangs off the back so he's got more space to swim and have better filtration.

Still in his current quarantine, i'm treating him for worms. But I also noticed some small white spots on his body, I think it's freshwater ich since he came from freshwater in the store and I only just started to increase his salinity (now at 1.003SG) to try and combat it since I don't want to mix medication in the water. The temperature has been raised to around ~27C to speed up the ich life cycle. What salinity kills ich? I want to drip acclimate him to higher SGs instead of waiting for a full week before increasing .002 again, and I don't want him still having ich when the tank is cycled and ready.

Tank cycling update:
Ammonia is going down, last night it went down to 1.5pmm but the nitrates and nitrites are still high and arent going down. Nitrate was at 80 and, and Nitrite was at 5. Consistently around 60 to 80 and 5 to 6. So I'm gonna do a water change and add some java fern & moss, didn't want to add in plants while the ammonia was high. I'll continue feeding the bac ammonia around 3ppm every day. Is there anything else i can do? and how much % of water change to help with the high nitrates?
Temperature wise is always at a consistent 24C to 25C.
There's also white-translucent algae on the glass and some decos, I think it's biofilm so I've just left it alone.

[Pufferpunk]

If there are no spots on his fins, what you are seeing are his tiny spines.

[piffpuffer]

There are two small spots on his top fin only, and one tiny almost missable spot on his inner tail. Been there for a few days even after I started the SG increase. I suspected the dots on his body might be spines but i started to doubt it when I saw the two dots on his fin.

[piffpuffer]

It's quite hard to get a picture of it cause he's always swimming around, so sorry for the quality. I've attached a couple photos

[Pufferpunk]

I wouldn't worry about 2 dots. Ich appears like your fish has been salted.