tah532 ...... jmicky41 hit the nail on the head.....
Nuisance hair algae, typically, is the symptom of bad or deteriorating water quality. Do you test your water often? If so, what are your nitrate & phosphate readings?
A fluctuating pH can also cause problems, brought on (typically) by lowe alkalinity (dKh).
What size is the tank and how many fish, and what type fo fish do you have?
What do you feed? How often? Do you rinse it first? Are you sure food is not going uneaten?
Do you have a clean up crew (if your fish selection allow)?
I presume we are talking FOWLR? If you have 1lb per gallon or more of LR, then why are you still running a wet-dry (surplus to requirements)? A wet dry (aka nitrate factory) will not do you any favours as far as algae goes.
Do you do water changes? How much and how often?
Do you have a sand substrate? Is it fine sand or crushed coral? How deep? Is it "live" (ie. full of worms adn pods etc.)?
What type of flow do you have in your tank?
What type of skimmer?
Do you have a macro algae refugium?
Sorry about all the questions, but I could type a 10 page essay on possible causes without this type of info.
Generally, you can have two types of hair algae, in my view (well there are 10's or 100's of types - I'm just generalising):
The type thats easy to get rid of and the type thats not...... the latter is more of a reefing problem, where algae persists in spite of pristine water quality...... we will assume that you have the forner.... the type that is being fuelled by nutrients.
Generally, the first thing yopu need to do is test for nitrate, phosphate, pH and alkalinity. People normally get away with alkalinity in a FOWLR because frequent water changes buffer it up..... but it is very easily fixed by adding baking soda (from the super market). You simply mix it with a bit of fresh water and pour it in..... its very easy and will benefit your tank in many ways - most notably by buffering and maintaining a higher, more stable pH. But - I would not add anything to a tank withtout being able to test for it before and after......
Nitrate and phosphate - depending on your fish, you could probably dramatically reduce levels by a series of large water changes, using RO water. First turn off all pumps then clean, srub, and remove as much hair algae as you can by hand. Let settle for a while then do a big water change, getting out any visible muck you can see....... anything up to 50% is ok, as long as the replacement water is aged 24hrs and adjusted for temperature etc. Repeat this every couple of days for a week to 10 days...... this should reduce nutrient levels dramatically, remove the roto cuase of it, and thus starve out the remaining algae.
This route is dependent on having good make up water, and to be honest, a 10% water change using RO water is as good as a 20% change using tap water....... so if you haven't got an RO unit, get one..... unless you have perfect tap water, which is unlikely and rare.
Doing this alone, together with buffering up your pH using baking soda should cure the problem. The next step is maintaining it.... which comes down to regular maintenance, good equipment, sensible stocking levels and correct feeding methods.......
HTH
Matt