Thank you all for your replies and concerns.
I've used the main tank, as he's only a new fish (had him 2 weeks) and he gets very stressy in the smaller tank. It's also a pretty new tank and I'm a bit scared of missing water changes. The temp is 80 degrees, which Sid loves.
I did some research on the net and found that water changes are fine right before it's time for the next treatment, as Protozin becomes inactive after around 24 hrs. Water changes also help rid the tank of any residual nasty swimmers released from the ich cysts, apparently. So I'm re-dosing after each water change, since it'd be time to add the new dose anyhow.
The ich spots are all gone, and so is the lump that one stubborn one left. I vacuumed the gravel thoroughly and did 100% water change, leaving the filter material (minus carbon, I took that out before starting the treatment as per instructions) and ornaments alone, as that's supposed to be where most of the good bacteria live anyhow.
My water quality is still really good, and I'm testing daily just in case. My fish is still a happy camper, and the last dose goes in tomorrow, and then I guess we'll see whether the ich stays gone or not.
I've read some really mixed reviews of Protozin - it either works a charm or really doesn't do a thing. It seems to be working so far, so cross fingers, I guess.