You really should go over to Container forum. It's the best place for container gardening including JMs.

Plain potting soil will eventually kill your JM unfortunately. What it needs is loose well draining soil. Look for Al's 5-1-1 mix in the container forum. That's what I use.

There's a lot of information about fertilizing there as well.

No need to visit the Container Gardening forum (although you certainly can) - there's a lot of participants on this forum that grow JM's in containers and can answer your questions :-) And Al, the container gardening guru, visits here often.

JM's in containers DO require a pretty barky soil mix - not what you typically find in prebagged potting soils. Where in Idaho are you located? Any garden centers that carry Gardner & Bloome or Master Nursery soil products? If so, look for their Acid Planting Mix. This is an excellent bagged product, recommended for Japanese maples and what I use myself. It is very close to the 5-1-1 gritty mix that was referred to above. Otherwise, check out that recipe - it is excellent.

Plain potting soil will not necessarily (very likely won't!) kill your maples. But it will degrade and breakdown relatively rapidly and need to be replaced more frequently than a coarser mix will. You have a choice between repotting once a season or once every few years :-) The gnats are most likely fungus gnats and an indication that you are watering too frequently or your potting soil is too moisture retentive. Very common in a lot of prebagged container soils. A coarser, more freely draining soil will prevent this but you can also back off on the watering and apply a layer of sand as a topdressing. This prevents the gnats from laying eggs in the soil. The larvae feed on the organic matter in the soil but they can also feed on tender roots. Not an extremely serious problem but one you want to control.

As to fertilizing, a CRF like Osmocote is used by many growers, applied once or twice a season. You can also supplement with a liquid fertilizer during the growing season. If your water is on the alkaline side, I recommend using a very dilute solution of MiracleGro for Acid Lovers (aka Miracid) wih every 4th watering. If not, any complete water soluble fertilizer will work.

Do you intend to keep your little tree in a container permanently? If so, you will want to pot up to larger sized containers gradually and do some research on root pruning. But that's down the road a ways........:-)