Are you using CO2 and adding ferts? I hope so at 12 hours your plants are going to need them. I have two 20gal tall and run 55w for 5 hours. But they don't have any CO2 and very little frets add to them. IF you add all this 8-9 hours is plenty. Most plant will only photosensitize for 8 hours then shut down even if the lights are on.
I would cut your lights back. 6-7 hours would be fine. Once you start adding ferts you can up them to 8 hours. Don't wait to long about adding ferts. With high light and CO2 their needing lots of food.
Lets say you just set up a new 20gal tall. You have 65w of light and plain gravel substrate and that's all. YOU add a bunch of plants stem and roots feeders like sword and Crypts maybe some Ferns and Anubias. With 12 hours of light stem plants grow like gangbuster. At the end of the first week swords have one new leaf and every thing else is still recovering from shock and getting accustom to the new light. Most plants shut down after 8 hours so the stem plant shut down but the lights are still on. Who is going to keep using up what little nutrients left in the water column? Algae!
After a couple of weeks there's so little nutrients left the nothing is growing now except algae. Algae will grow when there is to much ferts and light and not enough ferts and light. SO what to do? There is two way to go here.
1)Reduce light to 6 hours and add one kind of basic ferts ( Kent Por Plant in what I use) By reducing light you have plants growing in all of the light on time. Using up all the nutrients before the algae has a chance go take a hold.
2) Same thing but in steroid mode. Add CO2 with Micro and Macro ferts. Increase light to 8-9 hours maybe as high as 10 hours. Here is where you have to balance your light with the amount of CO2 levels in your tank. One thing I do to determine how long lights should be on is to watch for the brown algae that grow on the glass. I have found that you can increase or decrease you lights by just one hour and this stuff will stop or grow very slowly.
But point here is 65w of light on a 20gal tall make this tank set up a high end tank that need attention to water parameters and plant growth to keep algae in check.
If you want to see when plants shut down watch a plant like Ambulia. When this plant is growing you can see how all of its leaves are open wide towards the light. When this plant shuts down it closes up even if the lights are still on. This will show how plants only photosynthesis for a set time then need to rest.
thanks for the detailed answer Hawkeye. The only problem is, I don't want the fish in the dark for 18 hours or so, and I want the lights on for a while when I am home so that I can enjoy the tanks. dilemmas dilemmas
How does one measure how much CO2 is in the water? I see what your saying about growing like gangbusters, the cabomba has doubled its size in one week and is turning "purple" versus the bright green when I bought it. I will reduce the photo period a couple hours, I have eco complete as a substrate, don't know how much nutrients this stuff provides I'm buying ferts today, do I need to buy the individual stuff like iron , mang, etc. or is there a good all purpose fert out there?
Start out using ferts you can buy at your LFS. Like the Flourish line (Flourish, Iron, Nitrogen, Potassium, and Phosphorous) Kent make a line of ferts too like these. I still use these all thou I have been thinking about buying my ferts to save on cost. But to start off this is the easiest way to learn how to dose ferts. You may not need to dose all of these at the same time but at some time you will need some or all so I keep them all on hand.
You need to get some test kits. PH, KH, NO3, and PO4 are the basic and a must for me. I think I rely on test kits to much but they are a big help for beginners. Once you know PH and KH you can use the CO2 chart to calculate the CO2 rate in the tank. One of the most important things is to keep your CO2 levels at a constant rate. If it varies up and down allot that can cause algae.
That sounds about right. DO you have any brown algae on the glass? If so just adjust your light down my one hour. You are using CO2 24/7 right? I think I remember your stating so. I think it make a difference in light time by 2 hours. When you see post of people that have lights on for 12 hours, they are using a regulator with a solenoid. I run 24/7 and I can't run my lights over 9-10 hours.
Okay I've noticed some brown algae growing plus some green growing on my glass, Ive gone ahead and reduced the photo period to eight hours, from noon to 8pm. This Cabomba I've got grew a foot in one week, I wanted it to grow fast in order to help cycle my tank so I could have fish right away but not this fast, pretty soon it'll be a cabomba tank with a little water and stuff added. A read in a book that I should run a airstone at night for the fish to keep oxygen levels up , is this really necessary?