yes the exposed channels allow algae to grow. It's not too bad (as compared to earlier this year) so I'm taking my time preparing a fix, though I'm basically thinking I'll blanket a piece of black platic over the top to keep out the light.(just at the ends and return channels).
My troughs also allowed too much light into them initially which produced a hella lot of algae. I fixed that by wrapping aluminum foil around them.
I make my troughs out of pvc downspouts found at the hardware store. They were, I believe, $5 for 10 feet. The biggest problem I have with them is capping the ends. I use pvc gutters for my return trough with a piece of 3/4 dia. pipe draining it into my reservoir. Again, water tight seals is the biggest issue.
Set the troughs level, then raise one end with a piece of 2x4.

I use and am starting to hate round PVC tubes, unfortunately you can't buy PVC or Vinyl fence posts or square/rectangle down pipes in my country.

Why do you like the tubes BBrush?

you mean hate? They pool to much hence problems with root root, they are also to deep so by the time the roots on some plants are long enough, its time for them to be harvested! In saying that I did have great success growing winter veggies in them i.e swiss chard and the chinese cabbages grew great in them.

Yes, algae happens. What I get is a string-type algae at the ends of the channels and in the return gutter. I usually just go through an pull out whatever I can grab. I really don't think the algae affects to much...it is just some more plants growing in the system...lol

American Hydro now sells a covered return option.

There are also other manufacturers that sell a completely enclosed set-up where nothing is exposed to sunlight. The argument against these is that it is hard to see a blocked feed tube, so you may have a channel dry out and die. With the Am-hydro channels I can walk at the feed end and see if a feed tube is blocked (there are 2 per channel).

**SMACKS HEAD**
I thought you meant American hydro as in Americans who use NFT for hydroponics. I didn't realize there was a company called American Hydro. duh!
I wonder if their product is actually made in America?

@grizzaman hahaha thats ok! But...what's a down spout??

According to at their prices they may well be made in the USA...

Uphill Lipping (that's how it's called) versus closed with a (often cemented) lid depends on the scale. With a smaller setup you would/could control micro tube by micro tube nearly as quickly as bending down (or checking from some distance- if possible). With a huge greenhouse with hundreds of channels on the other hand, you would walk along the inlet side and check much quicker "optically" from the outside but wouldn't pull out a thousand micro tubes two times a day.

a down spout is the tube that takes water collected in a gutter to the ground.

I'll try and explain my system since I cant message you or link my website. Here is a pic of my test system

get a 2 7/8" hole saw or as close to 3" without going over.
I drilled out a line of holes in the pvc 9" on center. leaving about 3-4" at the ends of the pipes to keep water from splashing out.

I also bought 1/2" electrical conduit bulkhead fittings..they're plastic fittings with a threaded end on one side so you can feed it through a wall and attach the nut on the other side. you find them in the electrical section at home improvement stores. I bought the ones that have a 90 degree elbow. like this but mine have and elbow section on one side {{gwi:997785}}

I drilled one hole in the endcap and attached the bulkhead fitting, on the opposite side of the pipe, opposite the holes for your plants i attached another bulkhead fitting. i did this for each pipe.

then i drilled holes in the lid of the tote, one for an airtube line, one for the pump feed line and one for the return line. I ran 1/2" tubing to the bulkhead fitting in the endcap, and then connected each tube with 1/2 hose and finally a short section to drain back to the reservoir.

i start the seeds in rockwool and keep them in a shallow plastic tote under a florescent shop light, fed 2x a day until the root system is strong enough to transfer to the main system.

hope that helps and i'll add an article from urban garden magazine, step by step with pics.I tried the square tubes and didnt like them, the and caps dont fit tight and it's a pain to seal them

Here is a link that might be useful: DIY

WOW! Thanks! That's exactly what I was looking for!
~~Cheri~~