I would leave the soil alone. Ants prefer a dry situation so when you water, give the pots a good drink. Water once and then again about an hour later to get the moisture throughout the pot. Having said, you don't have to water every day.

Ants do not like the smell of lemon, so you could add some lemon juice to your water. Not too much or you may damage the plants as well.

Thank you very much. I’ve used the lemons from a gin and tonic - will the plants mind!

Shouldn't do, as long as it does not make the soil very acidic, which it will already be because of the ant's formic acid secretions.