• Ich medicine is really really toxic and a suspected carcinogen (can theoretically cause cancer).  Don't get it on your hands.  One of the usual ingredients is an organic dye that is really really nasty.  My stained carpet reminds me every few days how nasty it is. DON'T SPILL IT!

• If the instructions say so, halve the dose if you have scaleless tetras, loaches, etc.  No matter what else is in the tank.  These fish are really sensitive and there is a very good chance they'll die.  It might take longer to cure it, but so be it.  If you're halving the dose, pay extra attention to add each day's dose as close to the same time as possible.

• Some ich medicines are in a "safer" form and are already at a lower dose.  Do not reduce the dose if you're not told to (eg: Kordon's Rid-Ich+ does not reduce the dose)

• The life cycle of ich is complicated.  You have to treat every day.  Once the last spot has gone from the last fish, you MUST keep treating for another 3-4 days. If the bottle says "cures in 24 hours", don't believe it.  Keep going for the full 3 extra days (see below).  It is not worth a repeat outbreak by trying to cut corners.

• Ich cures work by breaking the life cycle.  Nothing can kill it while it is a spot on the fish.  Then they drop off the fish and grow in the gravel.  Nothing can hurt them here either.  Three days later, the cyst erupts into 300-1000 ich swimmers.  This is what the ich medicine kills.  Hence having to treat for the additional three days.  If you dont do that, you'll have thousands and thousands of them a few days later.

• Raise the temperature as high as your fish can stand.  Add air because the ich medicines reduce oxygen levels in the water, and so does the hotter water.  Raising the temperature speeds up the growth of the spots on the fish.  If you get it to 85F, they'll finish growing and drop off (to the gravel) within 24 hours.  (This is why some bottles say '24 hours' - this is only if you can maintain 86F+ and your fish dont die).

• Turn off the lights.  Bright light breaks down the ich medicine.  You might consider covering the tank if it is in a bright area for long periods of time (or part covering it or shielding it).

• One of the usual ingredients - malachite green - also has antibiotic characteristics.  It can kill flexibacter columnaris bacteria.  This is a good thing because your fish will be at extreme risk of getting infected due to all the ich injuries.

• Ich gets around by nets, water, etc.  If you are looking to buy fish, dont buy them from a place that has an active ich outbreak in one or more tanks, unless you know that they use extremely good contamination control (net dips etc).  Most places do not, they use the same net and bucket every time, spreading the infection.  Or they use a single sump filter system which means that each tank shares the same water.

• If you're crazy, you can raise the water temperature to above 96F.  That will kill the ich dead outright.  No life cycle considerations, no 3-day-wait, etc.  They'll drop off and die.  The swimmers and cysts will die too.  (I had a heater malfunction in a tank with ich - I tried to raise the temp from 74 -> 82, and it malfunctioned.  When I got home, the water had been in the high 90's all day and the ich was gone. Thankfully, my fish lived - yoyo loaches and neons.  This might have been because I had an oversized aeration bubbler in there at the time, running on overdrive.)

Please continue to do as they say, remembering that your 20 gallon tank only has 14-16 gallons of actual water. It sounds like you are doing what you should do. Ick doesn't just go away over night.

It is good that you are curious about what to do. However, You and I are also probably cursed with that XY chromosome thing where we only want to "follow instructions when all else fails." ;)

In the meantime, you should already have done a partial water change; made sure the tank is running at about 80 degrees F. Don't feed them heavily. You might feed them a couple of times a day, but make sure no food is left uneaten on the tank bottom.

Treatment should continue for a week. The free swimming stage of the ick should be killed off as they emerge from the gravel. In 4-7 days they should be gone.

If you do a water change, add medication only to the new water. So add two drops for the four gallons you add. If the manufacturer suggests re-medicating in a certain number of days, do what they say. Some of those Ick medicines contain medicinal dyes. Those dyes may get absorbed some by gunk in the gravel and even by the silicone sealant on the tank joints. Or it may break down some.

I forget if you have more than one aquarium. If you do, don't move equipment back and forth. :) It is so cold outside here, I have been rinsing suspect equipment off, after I'm done with the fish, and leaving that (drained) equipment outside over night.

Food for thought. Did you introduce a recently purchased fish w/o quarantine? Did the tank get chilled recently? Either of those might explain why you had that outbreak now.

All the best!
u.s.