I hade the same problems. Gasping fish due to excessive CO2 and excessive ferts killing shrimp and snails.

I changed from 0.95 watt/litres economy bulbs (< 30 uE/sec/m2) and dying plants and bad pearling to 0.8 watt/litres T5 lights with super-reflectors (>120 uE/sec/m2) sporting champange pearling even from HC at the bottom.

If should have kept the economy bulbs but have built some kind of super reflectors I would have gotten from 30 to around 50-60. If I just would have changed the bulbs to real good ones like T5 Triton/Aquarelle but no reflectors i would have gotten from 30 to around 80..

80-100 seems like the magic number for good growth and good pearling from most plants. Far below that you will have serious trouble getting anything to work despite fish killing CO2-levels.