I agree, actinic should be available for 18in. However, don't know about at a whole 55 Watts. I think that will require a VHO. VHO are sometimes hard to find for smaller than 24 in. It even took me a long time finding 24 in VHO for my tank. Best bet is doing an online search for VHO, then look for 18 in bulbs in that page. Does swf.com have any in their dry goods section?
good luck,
sam