We've used a lightmeter from Amazon to measure light output.
Does look like you could do with increasing the light; We get 1000-1200fc (so around 10k-12k lux) at the top by the light, 600-800fc (so around 6k-8k lux) in the middle, and 400-600fc (so 4k-6k lux) at the base of the tank. That's with a Jungle Hobbies advanced LED light.