Is this be too much light ?

i think hairgrass needs a bit more light than that to really excel. correct me if i am wrong, but i believe that you need a certain minimum light level with tanks [especially small ones] to be effective. i know i have never had luck growing hairgrass but that could be a number of different reasons.

 

A 12 gallon tank is too small for the "2 watts per gallon" rule to mean anything. You need more light for smaller tanks. But, 36 watts of an AH Supply kit is most likely enough for that tank. (I think)

 

T5 bulbs in individual reflectors provide more light intensity than T12 bulbs with flat, white painted "reflectors". So much more that any "rule" would have to be different for the two bulbs and all of those inbetween. If you start with about 2 watts per gallon of AH Supply light kits for a 20 gallon or bigger tank you would be in the ball park for having moderately high light intensity. Tanks below 15 gallons or so, need more for the same intensity. That is the best I can do!

My experience also tells me that a light fixture that is smaller than the tank - shorter bulb or with the light not spread out front to back - will have a much higher light intensity right under the light and lower intensity in the outer areas. So, a 40 watt fixture over a 20 gallon tank could have the equivalent of 3 watts per gallon right under it if the fixture was much smaller than the tank top.

I think this means you really need to do actual intensity (PAR) measuring in the tank with the fixture in question to really know the answer to your question.

 

Sorry to dig up this thread tom, just wanted to recap on it.

So in the instance of trying to grow hairgrass with this tank and this amount of light if someone where to fail growing it... while they were dosing EI and injecting co2.. and the plant does not grow... a full lush... lawn .. what could the problem be.