Glutaradehyde, the active ingredient in Excel, can be nasty in concentrated form. You are using about a 1.8% solution. That's pretty weak. I'm a biologist/professor and make up my own 1.5% glut solution from the 50% concentration stuff. When using the concentrate
, I use protective gloves, a fume hood, and eye protection. Having said that, I use the diluted solution in some of my tanks at home and I don't worry a bit when I get it on my skin or about the fish when I use it.
I just wash after using it. I used it daily at a 2x dose in my 55 gallon tank for over a year until I got a pressurized system and it works great! I'd use it again and I still use it for algae control.
Regarding Glutaraldehyde in general, here is my experience:
You probably will not see results within a single week, I'd say try it a month for a good test run. It's NOT exactly the same as CO2, this chemical increases growth up to about 4X normal, depending on light. Pressurized CO2 can increase plant growth up to 20X ! It's NOT a fertilizer, it adds to the available carbon for plant growth, think of it as similar to liquid CO2. You still will need to fertilize your tanks for best results and may need to fertilize more heavily than normal. IMPORTANT NOTE: a few plants are sensitive to Excel and don't react well:
), Riccia, Mosses.
Sometimes these plants do well with the chemical, or die back and recover, sometimes they don't recover...
My Vals died back, regrew finally when I double-dosed, and then they grew great!
Plants that seem most sensitive are those that don't grow emersed at all, the true "aquatic-only" plants.
Ok, back to your light question.
Can you run only one bulb? If not, then I'd suggest using a layer, probably 2 of fiberglass window screen material. Each layer cuts the light by about 40%. With two bulbs you have probably more than twice the light you really need. Try using one layer of screening for about a month, see how it goes with plant growth and algae. If you dose excel daily, you can get away with slightly elevated light levels but make sure you dose Flourish comp 2x per week. If you start seeing algae issues, try 2 layers of screen.
It's all trial and error. Even the rippling of the tank's surfaced will impact the amount of light the substrate receives. Worst case, get some Frogbit and let it grow until it covers the water's surface. Then you can probably run both the bulbs, no screen and not get too much light!
Have fun with it and don't be afraid of using Excel.