Yes, salt the tank. This is a long term treatment because the life cycle of ich at about 18C is several weeks. It might be better to use a modified Holtfreter's solution. The salts in these solutions are the main reason lab axolotls do not often get ich.
My axolotls permanently inhabit water equivalent to 1.25g salt per litre. There are formulae on the linked axolotl site but my recipe is.
Calcium Chloride 4g -current brand Kontrol Krystals Combat-dehumidifier crystals from a hardware store.
Table salt (Currently Morrison's Bettabuy) 40g
Magnesium sulphate (epsom salts) 10g-get it from a chemist-horticultural brands are sometimes heavily contaminated with iron sulphate.
Lo-salt 4g- a readily avaliable mix of potassium chloride with salt. (Morrisons do not stock it nearly everyone else does!)
The free running additives in table salt appear to be harmless.
Directions
Dissolve the Calcium chloride first in about 200ml water. It will get hot and is irritant, add it slowly and stir well. Then chuck in the other ingredients and make up to 1 litre.
(If you dissolve the magnesium sulphate first the calcium chloride will become coated by a layer of calcium sulphate and not dissolve-if you try to make up a more concentrated stock solution calcium sulphate will precipitate out!)
Finally get a tight closing container to store your unused calcium chloride or you will have a horrible damp mess next time you need it!
Use 25ml of this stock solution for every litre of water. If your tank looses water by evaporation make up with unsalted water. Use the solution for standard water changes.
You might want to use a slightly larger level of supplementalion say 35ml/L for the first month or so to give an equivalent salt level of about 1.75g/L.