Re: Anti-syphon fitting
yep, 5/16 is the odd size in anti-siphon hose barbs. I did a quick search and didn't find them. I know depending on the tank configuration, anti-siphon is required to meet Coast Guard regs.
How about using the larger 3/8" anti-siphon, a short length of 3/8" hose, then use a 3/8 x 1/4 male hose barb connected to a 5/16 x 1/4 female hose barb to reduce it to 5/16?
Actually, you can use the 3/8" anti siphon, and connect it directly to a 5/16" hose by first putting the hose in boiling water to soften it and it will stretch onto the 3/8 barb. Its only 1/16" inch difference, so if softening the hose just doesn't do it, take a file to the barbs and file them down a bit.
Otherwise, just keep on searching.