How do i determine I have enough light?
My LED, I think, say's it has 3475 lumens. That seems like very little when i did the calculations based on this thread:
HOW TO Calculate your tank lighting (in LSI)
My tank is 48x12x20 (approximate)
If it's not based on lumens or watts and surface area etc what is it based on?
Do I think you wasted your money on that light source? No. All light sources that give you adequate light (and that have a nice professional form factor) are going to be expensive. I spent $600 on three 24" TrueLumen Pro LED strips for a 20 gal aquarium. I liked their slim form factor, but it took three of them to give me the light coverage I was looking for. The unit you bought looks to use more and higher wattage LEDs compared to my TrueLumen Pro strips.
Although I think those calculation-style efforts are interesting, in the link you referenced, I think they muddy the waters and still only provide a gross estimate - if that. Ultimately, what you want to know is the PAR: which stand for "Photosynthetically Active Radiation" and describes how much of the available light contributes directly to photosynthesis. There are meters to measure this directly and they are used in greenhouse, commercial, and scientific applications. The meters run about $379 (from Apogee Instruments, MQ-200 Quantum) for the type that would be useful to you.
Many may think that you use the meter once when you get your light source and then put it away in a drawer. But I use it quite actively during the evolution of a tank, especially as the plants grown in and begin to shadow each other, and also as the light source ages. Also, I've found that my LED dimmer doesn't always come back on to the same light level. So, every week during routine maintenance, I use it to validate my target. It's also extremely useful when putting together DIY lighting or building your own hood, etc. Using the meter, you can choose to use ANY light source technology and you don't have to know a thing about it, just measure the PAR. Measuring PAR takes out all the variables such as internal tank reflection, hood design, water absorption, surface reflection, bulb efficiency, and on and on… With a waterproof probe, you measure PAR at the plants in the water or at the substrate… this will be very different than a dry reading.
Back when everyone was using primarily standard linear fluorescent tubes, they implemented the "watts per gallon" rule as a method of targeting your light. That may have worked at the time as a rule of thumb because everyone was using a similar lighting technology and it was fairly predictable. But nowadays, there are at least 6-10 different technologies and they all produce different photosynthetic light (and waste heat) for a given wattage; and reflector design plays a huge role for tube or bulb style light sources. LEDs are relatively new on the mass market scene and they come in so many form factors that's its really difficult to gauge how much PAR they will produce.
Many of these LED fixtures are designed for reef aquariums. And these can be too strong for low-light planted aquariums. But building a dimmer, or raising the light, or filtering it with a neutral density filter will take care of that.
In PAR terms, a good target for your beginner planted tank would be 50-70 PAR measured at the substrate. Will your light source produce this over the entire tank? I don't know… it has to be measured to be sure. If you get lower than 40 PAR however, then in your non-CO2 tank you'd be co-limiting both light and CO2 which is not good. Keep in mind that the PAR levels I'm suggesting are for "thriving" plants, not "surviving" plants. You can keep a plant at it's light compensation point where it only gets enough light to offset the energy it uses to photosynthesize and you'd get zero net growth… This is "surviving". By contrast, a plant that actively grows, produces new leaves, fights off disease, and reproduces is "thriving". As an example, 20-40 PAR might be sufficient for mosses, pelia, ferns and anubias, but not for many other plants. The carpeting plants like HC and glosso require higher levels of light, around 65-70 PAR, to grow compact and horizontal. Plants that turn red, may only do so with even higher levels of PAR, around 90-120 PAR. But again, a general good range for a broad cross-section of plants is 50-70 PAR.
I will look into circulation etc and see how I can use my powerheads to aid in that like you mentioned. One note I have 2 Aquaclear 70 HOB filters on either end of the tank that are cascading quite a bit of water into the tank. Is that enough water movement?
It probably would be better to add a cross-tank flow in addition to your HOB filters. The goal is to make sure that you don't have any "dead spots" in your tank. It will be come even more challenging as your plants grown in and hinder the water flow. To that effect, it may be good in a long tank to have a power head at each end of the tank. Side-mounted impellers similar to the Vortech MP series work really well too. Vortech's themselves are very pricey but are worth every penny. I guess the word of caution here is to not create so much flow that it taxes your fish!