running t5 lights and algea

[i have crabs]

id focus your energy on other issues like phosphates and skimming,
i know you say their is 0 phosphates but thier is a cycle of adding or creating phosphates as other things use it up which makes it a hard thing to control/monitor, if you feed frozen food test the water you defrost the food in also alot of other things that get added to tanks have phosphates.
if you dont have a ro/di unit get one right away, biggest improvement i ever seen fighting algae is when i started using ro water.
also a bigger/better skimmer wouldn't hurt but also wont be the cause in most cases,plenty of tanks run quite well with no skimmer,if you have a sump or area to grow some macro algae it is probably worth the effort to do so.