I don't use Osmocote for my water lilies. I prefer either the aquatic plant tabs or sometimes Jobes or Miracle Grow spikes. However, if I was to use Osmocote, I would choose one that is balanced, like a 14-14-14 or 10-10-10, or as close to that ratio as I could get. Too much nitrogen (the first number) may cause more leaves and less flowering. Wrap the Osmocote pellets in a tissue or bit of paper, bundle it tight, and push it into the soil about halfway between the edge of the pot and the tuber, and gently firm the soil back around it. The paper or tissue will degrade.

Generally follow label directions, or use less than that (instructions are generous), and don't double up on different fertilizers within the time frame. Most fertilizers contain the same basic nutrients, and a double dose of nitrogen can poison plants. How often to fertilize depends on the soil and whether the fertilizer contains fast or slow-release (organic) nitrogen, so follow directions. Using fast nitrogen, I fertilize until early October, which is about six weeks before hard freezes usually stop the roses here. Other people stop earlier. I would guess the guy you talked to was using 1/2 or 1/4 doses of Miracle Gro. This is an unnecessarily expensive and troublesome way to fertilize. Fast nitrogen will last about 4 weeks in warm, sandy soil or 6 weeks in average conditions.

I feed potted tropicals like begonias, fuschias, cannas, bananas, brugmansia, (plants that bloom and feed heavily)once a week with Miracle Grow. I have two kinds - one for blooms and one all purpose. Haven't killed anything yet, not with fertilizer anyway.

I would suggest 1/4 to 1/2 the recommended strength MG 15-30-15 every 2-4 weeks. Wish that I could give you exact figures, but most or many of us that container grow with artificial fertilizers do it by 'ear' or perhaps 'eyeballing' describes it better. You will learn by experience how to read leaf color etc for how often to fertilize. It is in my opinion much better to err on the side of under-fertilizing. Put in a larger pot to start with. Try to disturb the root system as little as possible. I would go to a one gallon container or at least half that.