Excess nutrients in an aquarium is only asking for trouble. I can't be certain, but I would surmise that the fish deaths were the result of excess minerals. And it certainly caused the algae. And it certainly caused the plant problems you've mentioned.
Heavy metals are toxic to all organisms; iron, copper, zinc, manganese and nickel are heavy metals that are required micro nutrients for plants but when overdosed can kill plants and fish. Seachem's Flourish contains these micro-nutrients, and the website says Leaf Zone contains iron and potassium. Flourish should not be used more than twice a week in a low-tech or natural setup. I've no experience with Leaf Zone, but it is interesting that an excess of potassium has been known to cause iron deficiency in that plants cannot assimilate iron, and by adding even more the problem only worsens. Personally I would not use Leaf Zone in my aquaria.
Mention was made of the EI method. I have frequently warned of the consequences of dosing ad hoc nutrients. Tom Barr states that this method is intended for high-tech aquaria where CO2 is being added and higher light is used. The balance between nutrients and light is very important in providing the necessary micro-nutrients to benefit the plants without harming them and the fish.
I suspect your choice of light is not helping this either. Both tubes are high in the blue, but plants also require red to photosynthesize. While this might work under normal levels of nutrients, providing more nutrients than the plants can use with lighting also not providing the basics is likely to cause trouble. I would remove the actinic tube and replace it with a full spectrum that has more red than what you now have.
There is a delicate balance between light and the 17 nutrients essential to good plant growth. It is fairly easy to achieve, but care must be taken to ensure it is not upset. Deficiencies in plant growth may be the only result, but in this case it also included fish loss.