First off T8 will grow plants and ultimately be just fine.

T8 Pros - Available everywhere (esp in the 4ft length like you have), Good efficiency rating, Lots of color temp options, Best bang for your buck when trying to light a whole rack.

T8 Cons - Smaller bulbs and fixtures can get pricey, The bulbs and ballasts will produce mild/moderate heat (heat hurts humidity and will warp acrylic)


So with that in mind the T8 also are roughly 75 lumens per watt where as a T5Ho bulb is roughly 100 lumens per watt. Meaning brighter light using less power, less power equals less heat generation.

For a tank your size. It is recommended you use 3 36" T8 bulbs to properly light the tank from front to back and penetrate to the substrate.

OR ou could move away from Traditional bulbs of fluorescent lights and look at LED. Here is a link for a 36" (which is the length of most 40G tanks) on Amazon. Very cheap and has everything you need for a well lit vivarium and healthy plants

Amazon.com : BeamsWork 36"-40" Single Bright Power LED Aquarium Light Fixture 600 : Pet Supplies

The pros to LEDs are longer lasting bulbs. Next to no power consumption. Very little heat production. Deeper light penetration in the tank. roughly 105 lumens per watt


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Ok now onto your tank top. Unless your 40 gallon tank has a brace running the width of your tank down the middle or has 2 supports an Acrylic top will end up being wildly frustrating. Trust me I fought and fought to use some cheaper method to make a lid for my 40B tank and after 4 failed attempts at a durable long last top I ended up buying the fish tank top I was avoiding all along. The acrylic will sag in the middle. Maybe not at first but with the heat and humidity in the tank over time it will begin to Bow. You can combat this by regularly rotating the tank top, but who wants to volunteer for a imperfect lid when better options exist.

This is the Lid I ended up getting and it was absolutely perfect! Of course this is the size I needed for mine. I am guessing yours is roughly 36" x 18"

Amazon.com : Perfecto Manufacturing APF34305 Glass Canopy Aquarium, 36 by 18-Inch : Pet Supplies

Hope any of this is at least some what helpful

Paul