With the amount of light you have you need to dose fertilizers more often than weekly. The plants will quickly use up the weekly doses, then languish until another dose is provided. The EI fertilizing method uses 3 times a week dosing, or even daily dosing, with lower doses, as I do. Other methods, such as PPS Pro use daily dosing. Also, be sure to dose adequate amounts, either per the EI method, which intentionally overdoses, or per the PPS Pro method which tries to more closely match doses to needs.
It is very hard to provide enough concentration and keep it consistent enough, using a DIY type CO2 system in that size tank. If you can afford it, go to pressurized CO2, and you will immediately see better plant growth results.