More about Ich:
1) Ich did not survive in your tank for 2 weeks with no fish unless it dropped to pond water temperature. 24-48 hour max at room temperature.
2) Do not combine medicines with UV. UV can deactivate meds. Use EITHER salt and heat (and you can add UV to this) OR medications.
3) Medicines lie.
Read about the life cycle of Ich, and remember this: Meds CANNOT kill ich while it is on the fish. Any medicine that says it kills in just a few days is lying. Ich has to fall off the fish to breed first, then the medicine kills the babies, not the adults.
4) You can separate the fish into different tanks according to which fish tolerate which treatment best. Loaches are fine with Rid Ich, but not so great with salt. Salt is not good for plants. You could also separate the plants from the fish, and treat the fish in a bare bottom 10 gallon tank with something the plants do not like (salt...). If the plants are in a warm, well lit tank while the fish are being treated, NO fish in the plant tank, then any Ich will die without a host.
5) If this is a 20 gallon tank a Yoyo Loach outgrows this tank really fast, anyway, so if you get another tank, make sure it is big enough for the Loach.
What other fish do you have?
Ich is an organism that has several phases to its life cycle. Only one of the 3 phases is vulnerable to salt or any other medication.
Killing Ich is a matter of understanding that life cycle and having treatment in the tank at the right time to kill the susceptible phase.
Trophont is the phase that lives on the fish. At first you cannot see it, but within a few days it grows to the white spot that gives this parasite one of its common names. It lives on the fish for several days (longer in cooler water) then falls to the floor of the tank. Medicines do not affect this phase because it is burrowed under the fishes' slime coat. Fish can be infected in their gills, where you cannot see the parasite. This is why a quarantine tank is very important: New fish live in quarantine from your established tank(s) until Ich or other diseases or parasites have been ruled out or treated.
Tomont is the phase that reproduces, most often on the floor of the tank. Diligent vacuuming of the floor of the tank can remove a lot of these, and I have even heard of complete cures in a bare bottom tank with daily vacuuming. Meds do not affect this stage. The Ich organism is in this phase for only a day or so at tropical tank temperatures. This phase releases hundreds of free swimming babies.
Theront and Tomite are the names given to the babies that swim in the water, looking for a host. This is the phase that is vulnerable to meds. They usually find a host in 24 hours, so keeping the meds in the water is important.
When you see a couple of white spots on the fish it may not look quite like full blown Ich at first, you are seeing the earliest visible signs. It grows quickly though to the obvious salt or sugar look. Since you do not know what ages of Ich are on the fish, and there is some natural variation in the growth rate, you can assume that you have all three phases in the tank by the time you see the first signs.
Treatment is aimed at finding something toxic to the Ich organisms without killing the fish. Some fish are more sensitive to certain medicines.
If you can raise the temperature the Ich will go through its life cycle faster, fall off the fish faster and reach the vulnerable stage (Theront) faster. If increasing the temperature is too hard on the fish then this is optional. Increase the aeration in the tank by adding a bubbler that increases the water movement, add a power head or other pump that creates more ripples at the surface, or drop the water level so the water returning from the filter will create more turbulence at the surface. If the fish are still gasping at the surface, lower the temperature by a couple of degrees. It is generally safe to raise or lower the temperature by about 2 degrees F or 1* C per day. More than this can add more stress to the fish.
In cool water tanks Ich may live for several weeks, and in a pond for several months. Fish kept in these settings generally are not tolerant to increasing the temperature, so the treatment will take longer.
More about temperature: It used to be that Ich could be killed by sustained temperatures over 86*F. The problem is getting the WHOLE volume of water this temperature, even under the gravel. (Bare bottom tank is easier). However, there has been another form of Ich discovered that is resistant to any temperature that is safe for the fish. If you can get the temperature up this high (over 86*F), you may still kill the Ich, but if it does not die, your fish may have 'Super Ich'. Super Ich is also resistant to the common Ich meds.
Medicines that treat Ich:
Salt. Sodium Chloride. Add salt slowly, over several days. 1 teaspoon per 10 gallons is a moderate dose, 1 teaspoon per 5 gallons is a stronger dose. Add the day's worth of salt to a cup of tank water and pour it into the tank slowly, over several hours. Next day, do the same. 3rd day, the same. At this point you will have 1 tablespoon per 5 gallons (strong dose) or 1 tablespoon per 10 gallons (milder dose) in the tank. Whenever you do water changes add the proper dose of salt to the new water so the level stays the same for the full course of treatment.
Salt can be the iodide- and anti-caking-free forms such as Kosher, pickling, water softener, or aquarium salt if you have any worries about iodide or anti-caking ingredients (read the labels to be sure). I have used table salt (with anti-caking ingredients and iodide) for short term use with no problem. Do not use Epsom salt, or the salt and mineral blend that replicates ocean water. This level of salt is bad for plants.
Rid Ich: A Kordon product, active ingredients Formalin and Malachite Green. (These ingredients are common to many Ich treatments.) Labeled to be a gentler medicine for sensitive fish. You can also ease the fishes' introduction to the meds by dosing similar to the salt: Add half the dose in the AM and more later in the day. There are instructions on the label for dosing weaker or stronger fish. Remove activated carbon from the filter. May stain silicone and some other stuff in the aquarium. If you use Amquel Plus dechlorinator switch to something else while using a Malachite Green based medicine.
Coppersafe and other copper products. These seem safe for the fish, but fish keepers who have used copper preparations have noticed that shrimp and snails also die, and the effects seem to linger for a long time, as if the copper has not been fully removed from the water, making it impossible to keep shrimp or snails in tanks that have been treated. Remove activated carbon from the filter.
Clout (long list of unpronounceable ingredients) by Aquarium Products. A much stronger medicine, not for use on delicate fish. Remove activated carbon from the filter.
Ultra Violet Sterilizer: Will kill the Theront phase. Safe for all fish.
Combining treatments: DO NOT combine medicines unless BOTH are labeled as safe for use with each other. Part of this is that 2 meds may combine chemically to form toxic substances. It can also be that each medicine weakens the fish just a little bit, but the combined effects of 2 medicines are more than a fish (already under stress from Ich) can tolerate. UV may deactivate some meds.
Here is one possible treatment schedule. I have dosed per sturdy fish in a 10 gallon tank.
Day 1: Water change emphasizing gravel vac (I am using the term gravel vac even if you have a bare bottom tank- same idea- do a water change that emphasizes the removal of whatever is on the bottom of the tank) Add 2 teaspoons of salt in a cup of tank water, spread out over the course of the day. Increase the temperature by 2*F. (Note the cautions above about aeration in the tank as you are raising the temperature)
Day 2: Add 2 teaspoons of salt over the course of the day and increase the temperature by 2*F.
Day 3: Water change emphasizing gravel vac. Add salt to the new water at the rate of 2 teaspoons per 5 gallons to match the current dose in the tank. Add more salt (2 more teaspoons) over the course of the day and increase the temperature by 2*F.
Day 4 and beyond: Continue water changes every other day (daily is even better) remembering to add salt at the rate of 1 tablespoon per 5 gallons to the replacement water. Continue to raise the temperature if the fish are handling it OK.
Continue treatment until at lease 3 days beyond the day you saw the last Ich spot on the fish. Remember the phases of the life cycle: A spot that falls off the fish does not become vulnerable to meds (actually it is the babies that are vulnerable) for a day or so after falling off the fish. The meds need to be there to kill these last babies. If you are treating a cool water tank keep the meds at therapeutic levels for a week beyond the day you saw the last spot on the fish.
Begin to reduce the salt and temperature SLOWLY. Do small water changes that do not contain salt.
If you have used other medicines do larger water changes and add activated carbon to the filter.
Reduce the temperature at the rate of 1*F per day.
Additional meds: Fish under stress tend to lose more body fluids than healthy fish. Increasing the total dissolved solids (TDS) in the water reduces this tendency. You can add salt (Sodium Chloride) at the rate of 1 teaspoon per 10 gallons, if the medicine you are using is labeled for salt water. This is not a high enough dose to kill Ich, but is generally safe even for fish that are sensitive to salt for the short term of treatment, and helps them regulate the amount of fluid in their cells.
I have heard of using antibiotics with or following Ich treatment, with the thought being that fish in a weakened condition may be more vulnerable to bacterial infections. DO NOT combine meds unless BOTH are labeled for use with each other. I have not had Ich followed by bacterial infections, but simply monitor the fish following the Ich treatment and be ready to treat for whatever else may show up. I do not think they need to be treated for some unknown disease that does not seem to be present.
Nitrifying bacteria and Ich treatment.
Some medicines can kill or weaken the colony of nitrifying bacteria. Monitor the water for ammonia and nitrite and be ready to do water changes as needed to keep the ammonia under .25 ppm and the nitrite under 1 ppm. When you do water changes for any reason during Ich treatment re-dose whatever medicine you are using to maintain the therapeutic dose in the tank. Once the medicine is cleared from the water you may want to add nitrifying bacteria to the tank either a bottled product like Tetra Safe Start or Dr. Tim's One and Only, or else add some filter media from a cycled, healthy tank. (Do not swap filter media: you might be adding Ich to the healthy tank, just get some media from a healthy tank and add it to the recovering tank)
Do not cross-contaminate healthy tanks and sick tanks. Use separate equipment on the various tanks, and treat the equipment to kill whatever disease or parasite you are treating in the hospital tank.
Ich does not survive without a host. If you lose all the fish in a tank I would run the tank for a few weeks with frequent water changes and gravel vacs and keep the bacteria fed with ammonia, similar to the fishless cycle. (add ammonia to the 3 ppm level daily until you are ready to re-populate the tank, then do as big a water change as needed to reduce the nitrate as close to 0 ppm as possible). Ich can live in the gills of fish, and can exist at a low population level so that you may not be aware that a fish has Ich. This is what is likely behind the thought that Ich can be 'dormant'. It has no dormant phase.
When any one fish in the tank shows Ich the whole tank has Ich. You cannot see Ich in the gills or on the floor of the tank, but you must assume it is there, ready to re-infest the fish. You may separate the fish to different tanks according to their medicine tolerances for treatment, all these tanks then become hospital tanks and need to be treated, even if you do not see Ich on all the fish.