I have found that too strong of light will also make it bitter as well as inadequate nutrients.

I grow most of my lettuce indoors year round and have not had problems with that system, but the system is 3 years old now and well aged.

Inside the greenhouse I had to overload things for a while to get the correct amount of nutrients in the gravel, the introduction of a pound of redworms to each tub helped out a lot too!

We are iron defficient here so I have a couple of horseshoes in the tilapia tank