A few things.
First, I am glad you are going with a 20 long.
Whenever a larger tank is mentioned in the marine world, there is at first some resistence due to price. We all understand this, surely. But most of us would also acknowledge that the smallest part of your investment will be your tank. At least at the nano stage. The extra few bucks you spend for a larger tank will pale in comparison to what you will be spending elsewhere. And it does ot really cost anymore to purchase good lighting for a 20 gal than a 10.
On the waterproof/resistant front, you have two options for lighting.
With budgetary concerns, regular flourescent are the way you are going to have to go, as you have surmised.
There are companies that make waterproof endcaps for light tubes. This allows you to put your lighting closer to the water line without much fear of corrosion or shorts. I have them, and they work very well, but, there is always a but aint there?
They are expensive, and only work for a specific tube diameter. This means that if you start out with NO, Normal output flourescents, and then later wish to upgrade to VHO, very high output, the VHO tubes are smaller and your pricey endcaps won't work.
What I would do is buy a four tube electronic ballast rated for three 4ft tubes. This ballast will have an output capable of 120 watts. Wire that ballast to three 2ft tubes. This is called ODNO, overdriving normal output fluorescent. I have done this to NO bulbs up to three and four times their normal wattage with great success. Even with the inefficiency of overdriving you should still have an output of over 100 watts. This will allow you to mount the lighting higher up in your homemade canopy and avoid most of the salt splash and creep, thus eliminating the need for the pricey endcaps.
What it will come down to is the DIY skills of you and any friends that are good with tools and hands. That will determine success on a budget.
GL
) side of the pond from me, I really can't recommend a specific ballast number. But Advance makes good reasonably priced ones if available there. Any lighting place should easily be able to help. They will aslo carry the simple, normal standard light end connectors for about a buck apiece, normally less than that.
