I have something going on with my rainbows, but I'm not entirely sure what it is. Initially, it is just the white/hazy patches circled in the photos, behavior and eating habits are normal. Eventually, the patches grow, the fish stops eating and hangs out in a corner at the top of the tank, and (at least in one case) they eventually die. These two are the 2nd and 3rd fish in this tank to develop this issue, the 1st passed. The more "clamped" looking one was starting to act a bit off this morning, the other is still acting completely normal.

I started treatment for columnaris on Saturday using Spectrogram (kanamycin/nitrofurazone), but don't know if that's what I'm dealing with and so far see no improvement, but I've only done 1 of the recommended 3 doses thus far. The tank runs about 77 degrees, ammonia and nitrite are 0, nitrates were 20 prior to Saturday's 50% water change (haven't checked since), ph is 8.3 (which is what it is out of the tap and I don't jack with it). Tank is lightly planted and low tech.

Does anyone recognize this as something besides columnaris and/or have treatment suggestions?

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It looks very much like columnaris (along with the other symptoms you describe) and it will take more than one dose to see improvement.
It appears to be the slower variant with the symptoms, there are 4 strains of this disease.

I had the same thing on my harlequin rasboras. I treated with Paraguard for 14 days, and they were cured. They're still in my tank. I don't know which fish disease/fungus/parasite it is, but the nice thing about Paraguard is that it treats almost everything. Just be sure to do the full 14-day treatment, and remove any carbon or purigen from your filter. You can give the fish a bath, but that won't do anything for whatever is floating around in your tank, so I'd treat the entire volume. I'd also dose StressGuard for the full 14 days.

Seachem recommends not using Paraguard with inverts, but I've read many posts saying it doesn't harm them if you treat the tank volume. It will definitely harm inverts if you put them in a concentrated bath. If you don't have inverts, there's nothing to worry about.

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Thank you (I think! lol) for confirming my suspicion. This isn't something I've (successfully) dealt with before - is there anything you'd do differently as far as treatment goes? Is the 3 doses the drug recommends usually sufficient or should I plan to extend it further?

You are treating exactly how you should be. Just make sure that every 3 days when you dose with the kanamycin that you do at least a 25% water change before adding the next dose. The less organics in the water the more effective the medication is.
Do you have an air-stone in there while medicating? This is important to.
It is not unusual to have to do 2 rounds of treatments for Columnaris. You should be able to determine by the third treatment is over if this is needed. If the cloudiness clears up, the fins are no longer clamped to body and, most importantly, they begin to eat again, you should be good with one round of meds.

Bump: If your fish did have Columnaris, Paraguard has no active ingredients capable of curing the disease on its own. But, fortunately whatever your fish did have, the Paraguard ( acriflavine, malachite green, and metheylene blue) was exactly what you needed.

I guess my fish had something else, but it looked just like the pics in this thread. Good to know, though.

Thanks for the information. I do not have an airstone, though I can grab one. The outlet of the filter (eheim 2217) is currently sitting a bit above the water line though and providing some surface agitation. I have enough meds on hand to do 4 treatments so I'll go ahead and grab some more in case another round is needed. Wish me luck! lol!

I think that the surface agitation from the filter should be more than enough. You can try and make it so that the water level is a tiny bit lower so that there's more agitation from the filter rather than spending money on a filter stone.