Tired of low yields

Feed more often
Coco should never dry out- as soon as it drains- there is approximately 20% air in the pot. I feed 2 x daily.
Top the coco with a couple inches of play sand or diatomaceous earth to get rid of the gnats. The BTI mentioned above is good also - both sand and bti will kill the gnats.
treat house plants with the bti a couple times also and every so often

spinosad kills gnats to death, I've never had an issue with them for long. springtails on the other hand are extremely resilient, but harmless and actually beneficial (they eat algae and also fungus gnat larvae). you can reduce their numbers by adding a bit of surfactant to your feed (ie castille soap) and some will drown, but I'd just let them be.

Greenman68 said:

spinosad kills gnats to death, I've never had an issue with them for long. springtails on the other hand are extremely resilient, but harmless and actually beneficial (they eat algae and also fungus gnat larvae). you can reduce their numbers by adding a bit of surfactant to your feed (ie castille soap) and some will drown, but I'd just let them be.

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I've never had a problem with those damn gnats and if I'm not wearing my glasses I can't see those fuckers...so finally I'm wearing my glasses and see these fuckers flying around...had been trying to figure out why my plants are for shit this time....well I have a ton of soil I male to run...but this time I bought soil to show a friend how to do it and use top dress organics instead of bottles etc...well the one bag of soil when I checked it again is full of those fucking bugs...lesson learned...look how bad it affected a new clone in about 7 days...yep...spinosad for the WIN!

ah bro there's zero chance that was because of fungus gnats, your soil is too hot.

Greenman68 said:

ah bro there's zero chance that was because of fungus gnats, your soil is too hot.

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Nope...bugs...it's already turning around.

nah that ain't bugs

Some simple things you can do.

Stop stripping all the leaves off.
Use a metal halide in veg to give shorter internode space. You'll have a shorter tighter plant allowing light to be closer.
Make sure your HPS bulb is not old or a shit brand, same for ballast.
Water more frequently
Pick a heavy strain.
Scrog.

In the long run buy a light that's suitable for 16 Square feet.