I have a few plants that are plants that I had for years and years and never had a problem with about a year ago I got these little bugs in them that looked like knats and then I changed the soil but over I would say about a month even quicker then before they were back and I have no idea what kind of potting soil to buy because the soil I got before had sand perlite and other things in it and was not the right soil. I have tried miracle grows Scotts and so on and nothing works I really don't want to get rid of these plants if you could let know what kind of soil to buy and where to buy it is the good question it would be greatly appreciated. PS the plants are in medium to large sized pots and there are three of them and one in a small pot which is a cactus thank you!!!!
Do you know what kind of plants they are?
Are the insects flying or staying on the plants?
It sounds like you could have fungus gnats, which is often caused by overwatering. If you let the plants dry out they generally go away.
That said, different plants may require different planting materials.
Are you able to send us clearly focused digital photos of the plants and the pest you are seeing?
Treatment will hinge on correctly identifying the pest.