Well first off, watts per gallon is a very old myth and is not correct at all. All wattage is, is a measurement unit of how much electricity the bulb is using. Yes, to a degree, the more watts true brightest but it is not about wattage at all. What you want is a PAR rating. PAR is parabolic aluminized refelctor rating. Basically, how well is the light being reflected into your tank and the there are two other factors; what is the kelvin rating and how close are your lights to your substrate.
So take a 15 watt cfl that has 6,500 kelvin and put it on two tanks. One tank that is 12 inches deep and the second is 18 inches deep. Youre going to get a high light situation, or higher, on the 12 inch than you will on the 18 because the light is closer to the substrate and can penetrate the water easier. Does this make sense?
Now for kelvin, ideally you want 6,500 or 6,700 as that is whiter light and close to what the sun is at. However you can also user high kelvin like 10,000k which is more blue light. Plants tent to use more of the red spectrum of lights so if you look on finnex's fugeray planted+ you will see theory have 6,700k white light s along with red lights to help the plants grow.
So in your 3.5 you will actually have a decent medium-high light situation! The 7 could be the same but I would aim for more medium plants
So take a 15 watt cfl that has 6,500 kelvin and put it on two tanks. One tank that is 12 inches deep and the second is 18 inches deep. Youre going to get a high light situation, or higher, on the 12 inch than you will on the 18 because the light is closer to the substrate and can penetrate the water easier. Does this make sense?
Now for kelvin, ideally you want 6,500 or 6,700 as that is whiter light and close to what the sun is at. However you can also user high kelvin like 10,000k which is more blue light. Plants tent to use more of the red spectrum of lights so if you look on finnex's fugeray planted+ you will see theory have 6,700k white light s along with red lights to help the plants grow.
So in your 3.5 you will actually have a decent medium-high light situation! The 7 could be the same but I would aim for more medium plants