even though you have a good flow, you may have a bad circulation. Generally though, these problems occur due to poor water quality and/or other maintenance issues including co2/fert problems. Remember, even though your using a drop checker that shows everything ok, it doesnt mean that co2 is reaching all parts of the tank. so check and double check.
You dont need to worry aobut aeration during a blackout. Obviously leave the filters and heater on, but turn off everything else.
Blackout procedure:
1. Carry out a 70% water change.. during this remove all signs of algae and cyan, scrape the glass, prune back any infected, weak or damaged leaves.
2. wrap the tank in black bin liners.. taping these down so as not to let EVEN A SMALL AMOUNT of light in. Then cover the whole lot with a large blanket just to make sure.
3. leave the tank for 3 days. Do not peak, feed or anything else. Any slight break in this will mean you have to start over. Dont worry, your plants will be fine.. in fact.. youll be amazed at how much theyve grown when you un wrap.
4. After the period is over, unwrap. Switch everything else back on and carry out another thorough water change/maintenance session.
then youre good to go.
You must make sure that you do everything to try and remove any causes, or it could reoccur.
Yes you have a good flowrate on your filter, but this is never the actual flow rate.. its effected by many things such as; length of hosing, filter media, how regular your filter maintenance is, obstacles in the way of the flow, age of the filter etc. Extra powerheads around the tank can help push water in/out of deadspots, as can clever scaping.
Blacking out really isnt a scary thing
its one of the best ways to 'reset' your system if its in dire troubles. But you have to be disciplined, be incredibly thorough in your covering, even a tiny bit of light will be enough to sustain spores/bacteria. You need to remember that algae/bacteria are a much smaller organism than a plant, therefore, they have no reserves like a plant does.. its this feature that youre exploiting with a blackout.
good luck, and let us know how it goes.