It is probably sun. I have Rozanne's planted in several different lighting situations in my yard. The ones planted in part shade are thin and somewhat leggy. The ones planted in full sun are full and thick. I recently purchased some Rozannes from a local nursery that had them growing in a shaded hoop. They were very leggy. But, a few weeks in the sun and there is thick new growth emerging from the crown.

What kind of fertilizer do they need and how often?

Ral, as with many annuals, geraniums in summer heat begin to look ratty. What we have to do is make them want to do better for themselves. Where the legginess shows, just pinch or cut back and make the plant want to replace that part.

Pinch off the long and wilting leaves and blooms. Give the plant all the sun you can.....but 6 to 8 hours is a must.
Water the plant ....if a hanging basket, about every day..and always to the draining point......garden beds can get along better.
Fertilize with any 15/30/15 about every 10 days to 2 weeks.

And let's not forget, before frost hits geraniums, they can be taken indoors to further their blooming time....Or...
can be considered for wintering over and bringing back next spring....as good...or better than ever.

Ralgam, I believe that caroldiane, is referring to pelargoniums, the annual geraniums, not hardy geraniums. As for fertilizer, I just use a timed release fertilizer.