Well, I know that you need higher WPG in smaller tanks, But I don't know why.

Lol.

I found this http://www.fitchfamily.com/lighting.html , But I don't really understand it.

So could anyone enlighten me on how it works to make us need more WPG to reach Medium or high lighting in smaller tanks apposed to larger tanks? I'm really curious on how it all works .

Thanks in advance~.

I have no idea either but I'd guess and say it has to do with the bulb.. Small bulbs produce less photons than bigger (higher watt) bulbs perhaps.. So it's very common to have 20-30 watts over a 5 gallon tank. That's 4-5wpg but that doesn't mean anything in real life.

Hm, I guess that would make sense!

Just trying to get a real good grip on all this before I start my second planted tank :].

Here's another article that might clear things up: http://www.rexgrigg.com/mlt.html

Thanks, reading over that whole site now, very informative :] .

This is what I get from that rex grigg article:

You should use the surface area of your tank to calculate the lighting instead of the volume, and going from a large tank to a small tank, the decrease in surface area is not as great as the decrease in volume, so your lighting needs should be greater than what WPG tells you (that's why you use LPG). Is that correct?

Watts work when choosing bulbs because with fluorescent lighting, watts are generally proportional to lumens. Lumens is light output (brightness), watts is energy consumption (your electric bill).

Bulbs are categorized by watts
Tanks categorized by gallons

So watts per gallons make life easy, because its a guestimate.

Yeah, I'm just gona loosely follow WPG.

According to http://www.fitchfamily.com/lighting.html , I should have 90 WPG in my upcoming 20 >_> .. I'm gona probably get more like 60-70ish.

Yep, that's what I get from it! Basically, as someone said, lumens are somewhat proportional to watts, making WPG a good estimate, but you need to nudge the estimate a little when you're dealing with the top and bottom ends of tank sizes. I don't think it's necessary to calculate a precise lux value; just knowing that "small tanks need more wattage so 5 wpg isn't going to fry the plants in my 5.5 gal" is good enough for me.