Murky Water Problem.

[rabbut]

Water is invisible and the fish are floating in air now .

Thats good to hear

It seems that after the white cloud has gone away the NitrAte level has gone up to 6-7mg/L. I'm not exactly sure what an acceptable level of NitrAte is in mg/L but my assumption is the lower the better. At what point should I be doing a water change?

Acceptable levels of nitrate, is anything below 50ppm. Anything above 50ppm is harmful long-tearm, anything above 100ppm is harmful short-tearm. I normaly waterchange weekly, 50% (very heavy stocking) and nitrate is kept below 25ppm. I should point out that plant growth starts to stagnate past 40ppm. Your tank looks to have malawi cichlids in, so you should also have above adverage stocking density. You need to do sufficent waterchanges to keep nitrate below 25ppm, IMO. This probibly means weekly 50%, like me, or two 50% weekly. The frequence and regularity vary depending upon test result.
HTH
rabbut