Check the roots for Vine weevil grubs.

I would say that gowing even a dwarf Malus in a container will mean that watering too much or too little will always be critical compared with planting in the ground. As Owdboggy says you also make your tree more open to the dreaded vine weevil. As you tree must have developed a good root system by now, I would bite the bullet and plant it in the garden.

When you say there are ants, what does that mean exactly? Are there ants on the foliage? Or lots of ant activity generally? If the pot is standing on slabs or hard surfacing, turn out the plant and check the rootball for ants nest. If you see loads of ants when you do this, run... get a large bucket or container, fill it with water, go back (after half an hour), stand the plant in the container of water, leave it there for at least 2 hours. Take it out, empty the water and ants out somewhere, put fresh water in the container, stick the plant back in again, and this time, sloosh it up and down and side to side to try to get off the compost, or as much as possible. When you're sure you've destroyed any nest that's in there, repot into fresh compost, water thoroughly. And this time, stand the pot on pot feet or broken roof slates, anything that gives a gap between the pot and the paving beneath. Dust beneath periodically with ant powder.