plants are turning brown/black...help!

they started out as little yellow spots with leaf tips curling down and have progressed to this. i flushed them yesterday with RO water and the last bit of flush was 1/2 nutes. any ideas before i lose them??

Looks like your not feeding your plant nutes. Could be nute lock if you are. Make sure to PH your water.

Magnesium (Mg) - Interveinal chlorotic mottling or marbling of the older leaves which proceeds toward the younger leaves as the deficiency becomes more severe. The chlorotic interveinal yellow patches usually occur toward the center of the leaf with the margins being the last to turn yellow. In some crops, the interveinal yellow patches are followed by necrotic spots or patches and marginal scorching of the leaves.
Calcium (Ca) - From slight chlorosis to brown or black scorching of new leaf tips and die-back of growing points. The scorched and die-back portion of tissue is very slow to dry so that it does not crumble easily. Boron deficiency also causes scorching of new leaf tips and die-back of growing points, but calcium deficiency does not promote the growth of lateral shoots and short internodes as does boron deficiency.
Potassium (K) - Older leaves show interveinal chlorosis and marginal necrotic spots or scorching which progresses inward and also upward toward younger leaves as deficiency becomes more sever

so in order to fix nute lock i need to give it a good flushing with ph balanced water?

What is it in (medium)? looks like a root system or ph issue.

Can we get a pic of the whole plant mate, along with your grow info would be great

its under a 400w hps in a 2x4 closet with a 4 inch inline fan w/ filter. The temp is a little high (80) as I do not have central ac and its is 90+ degrees outside everyday. I do have a portable ac in the room but the temps are getting kinda hard to control. They are grown in ff ocean forest and are fed ff big bloom and tiger bloom. I water twice a week or as needed with full strenth nutes. they are now into the first week of flowering. I flushed them twice and it seems to have slowed the problem for now. i am going to start back with full strength nutes because i see a couple leaves starting to get nitrogen difficient. i will know more here in a couple days wether or not this has resolved the problem. The lights are off for today and will try to get some pics tomorrow. I dont see how it can be a ph problem as i used only reverse osmosis water.