pH: 8 (degassed)
GH measures the presence of calcium and magnesium in the water.. Plants need calcium and magnesium to grow. If your GH is zero that might explain it.
KH measures Carbonate (CO3) ions in the water. The most common carbonates in aquariums are sodium and potassium bicarbonate and Calcium and magnesium carbonate. The GH and KH tests will detect calcium and magnesium carbonate. However sodium and potation bicarbonate will only register on the KH test.
Do you happen to have a house water softener, that could explain your KH and your GH readings. Plants and shrimp need calcium and magnesium to live and grow. IF the water is too soft both could die. Most fertilizers don't have any calcium. So if you don't use a GH booster your plants won't do well and might die
When you were using the GH booster what was your measured GH? Also what brand GH booster did you use? Did you discontinue the GH when your water turned cloudy?
I would suspect the plants melt you have and shrimp die off were probably caused by a low GH level.
Also note your PH of 8 is not compatable with Fe DTPA in your Thrive fertilizer. If the PH is above 7 the Fe DTPA will start to break down and may as a result note be available for plants. In This case there are only 2 types of iron supplements that will work with PH 8 water.
Fe EDDHA, This iron fertilizer salt can tolerate a PH level much higher than 8. But strongly imparts a red tint to the water and is not frequently used in aquariums.
Fe gluconate,this iron salt is not affected by PH but the gluconate ion is a form of sugar and will be rapidly consumed by bacteria. As a result it should be dosed every couple of days to maintain a sable level of iron in your tank. Seachem iron is an iron gluconate supplement.
Lack of iron with a low GH could also contribute to plant melt. At this point I don't think your dimmed light caused the plant problem. And I doubt CO2 was the cause of your shirmp die of.