I don't really understand your question since you already seem to have answered it by doing the math to get the 1.4wpg that you did. IMO, though, that isn't enough and you'd need more than that for your planted tank...and...I suggest you change the bulbs to within the 6,700K to (no higher than 10,000K) with closer to the lower end of that range being best.
Shadow, 1.44wpg is really too low, IMO, to grow what you want to. I don't know too very much about "low tech" planted tanks, but as far as I understand, that amount of lighting is even too low for that, but you should look into "low tech" planted aquariums to get more detail on that.
I'm looking to add another light. I just took the tank out from the basement [yesterday] and put my light from my 55g on. I was just wondering how what I was at for now. But I do plan on adding more light. This is the first time the 75g has been set-up as a planted tank and I know I need to buy a additional light for it.
So how many more watts do I need to get at least to the [mid] medium to [low] high range? Im look to spend no more then $100 on a additional light
And thanks Ill get different bulbs. I was kind of wondering
Well, I have a lot of light on my tank and I use Estimative Dosing with pressurized CO2, but people that are expert at this also say that you can keep a fine planted tank with no more than 2-watts per gallon, so I wouldn't doubt that. I'm reluctant to give you any advice with that though, because until I increased the lighting on my tank, started to use CO2 and do the EI-dosing, my success was sporadic.
So, if you don't want to go the route that I did, I'd say that you still can have a fine tank so, look into "low tech" planted tanks like I had said. It's a technique that might be just what you're looking for and you can start here....
That particular brand has not had good reviews, from what I have read. It may not be a typical T5HO light. Typical T5HO lights can't be judged by watts per gallon. They light up the area under them to a pretty consistent light level, depending on how far the light is from the area where the intensity is measured, no matter how many gallons of water the tank can hold. A standard 75 gallon tank is 48"L x 18" D x 20" H, the same as a 55 gallon except more front to back depth. One single T5HO bulb can give enough light on a 55 gallon tank to grow just about anything. For the extra 5" in depth the 75 gallon tank has, a second T5HO light can give a more uniform intensity from front to back of the tank, but will also give significantly more intensity at the middle of the tank. For example, a 2 bulb Catalina fixture will give about 120 micromols of PAR in the center of the tank, and that is in the high end of high light intensity. But an Archaea 2 tube fixture gives about half of that - comparable to a one tube Tek Fixture. I suspect the Oddysea fixture would be no better than the Archaea fixture.
So, in my opinion, you can grow the plants you mentioned, with no problem, using just that Oddysea fixture, but with bulbs of no more than 10,000K color temperature. You might even need pressurized CO2 to avoid algae problems.
If I may, what Hoppy's suggesting is that with better bulbs than the 12,000Ks you have, you might even need pressurized CO2 so as to support the amount of plant growth your present fixture would foster so as to be able to successfully "out-compete" unwanted algae for the nutrients and light in the tank. I’d say that this would bring you outside of “low tech” territory.
I'd add that, at least from my experience, once you do add the CO2 you're going to need to support that growth potential with fertilization.
Well, then, I'd venture to say that unless you have the kind of experience that Hoppy has, if you do get the better bulbs, you might initially run into some problems. Once you add CO2, it isn't a simple matter to arrive at a balance of the three major elements of: light, CO2 and ferts when your tank isn’t heavily planted with fast growing plants and I’d say that it’s more than likely that you’d have some run-ins with algae and consequent poor growth of your plants until you can balance the tank correctly. I'd also say that you'd be quite limited in the kinds of plants that you'd be able to grow with just the two-bulbs.
But, at the same time, Hoppy's experience would tell you that it is possible to get it right and have a successful planted tank with the bulb change he suggests.
Iv had this light for over a year, no problem and have yet to change the bulbs.
I do have a full 20lb CO2 tank and ferts from the heavily planted 55g I had, I just need recommendations on lighting this bigger tank.
So would just buy the new bulbs be enough get around medium lighting. Or should I get a supplementry lighting.
Im looking to spend around $100 max on a lighting [$50 would be better]. The 75g will only be used a year or maybe longer. Im just waiting to finish the basement to set-up my 6' 125g tank. Then Im forking out the money
Your bulbs are probably just fine as is. If you don't like the LOOK of the light, get new bulbs, but I suspect what you've got will grow plants just fine.
I owned one Odyssea fixture and won't ever own another, personally, but if it's working then I'd stick with what you've got till you need to replace it.
And, as far as wattage goes, you've got perfect level for your tank with no CO2. You add any more and you WILL need pressurized CO2.
I run 2x54 watts T5HO over my own low tech 90gal and have to be very careful with the photoperiod; if I run the least bit over 8 hrs/day, I get algae. Currently, I'm down to 6 hrs/day since my tank is getting some sunlight this time of year...
I don't know how good the Odyssea T5HO fixtures, are though; my fixture is a Solar with really good individual ones. Reflectors can make a huge difference.