Water soluble fertilizer is that which must first be mixed with water and then poured over the soil.  It is the most immediate and effective fertilizer that you can use for your Hibiscus. 

A fertilizer with a lower middle number (phosphorus), such as 20-10-20 which I recommend, will keep your tropical Hibiscus happy and healthy. 

It will be tempting to use a fertilizer with a high middle number such as super bloom type fertilizers for more flowers, but for the long term health of your Hibiscus please don't and it doesn't increase blooming anyway.  Both the example I have given above, 20-10-20 (a true tropical blend), or 10-10-10 (a balanced formula) will do nicely.  The important thing is to feed them. 

Feeding your Hibiscus every week to two weeks is adequate to keep your Hibiscus healthy and flowering.

It is my opinion that time release pellets, such as Osmocote type fertilizers, are not as effective in giving your Hibiscus all the nutrients they need.  The amount of fertilizer released is dependent on the temperature of the air and soil.  The hotter it is, the more fertilizer released; the colder it is, the less fertilizer released.  Making it difficult if not impossible to know how much fertilizer your Hibiscus is receiving. 

The benefits of time release fertilizers is that they are easy to use and last from 4 to 6 months.  The plants are fed every time you water because as the water goes over the pellets, the pellets release fertilizer. 

The time release pellets are an excellent choice for those who don't want to bother mixing fertilizers or can't remember to feed their plants on a regular schedule.  Though they are not as effective, your Hibiscus will have some fertilizer and that is better than none.