YA CO2 thread - inconsistent results determining CO2

A little bit of history:

About 2 weeks ago, I had my CO2 running pretty high with a green drop checker (every once in a while there are hints of yellow-ish green). The pH was ~6.1 with my pH meter on the CO2. dKH is generally around 5.

The fish were showing signs of O2 deprivation, though not the normal ones of panting on the surface. They has slowed down quite a bit over the previous few weeks, didn't eat as much, and often would hide. My cardinals were slowly dying off.

When I went to add new fish, the acclimatizing water was like poison to them.
Everytime I added 1/2 C of water, they would head up to the surface. When I initially put them into the tank (for they were not happy in the bags), they never left sucking the water-air boundary. These fish were promptly taken out and put into a fresh 5 gal bucket with tap and prime. Immediate improvements.

I ran out that night to get a 20g tank to put them in (and to finally set up a quarantine system). I also picked up my first set of air pumps/stones.

When I added the new airstone to the big 75g tank, all the fish came alive! Within 30 minutes, with a .5 point pH rise as I gassed off the CO2, everything was looking good.

As I've tried to figure out what was going on, through various discussions here, I believe it was a combination of CO2 plus lower O2 due to my efforts to combat a breakout of hair algae.

The week+ leading up to this was a campaign against hair algae. During this time, my total light on the tank was reduced, CO2 increased. A few things had happened - When testing the ferts, it appeared that I was NO3 limited for I could never get a NO3 reading greater than about 2-5ppm. I also had a big algae die-off which significantly increased my BOD. (though I don't know where all the NO3 ended up going, unless my plant take a lot of NO3 and very little phosphate, for that was at ~10ppm). I was dosing PPS-PRO daily at that time.



Fast forward to today. After a few set backs (my filter lost the ability to move water), I am back running CO2 with a new filter (while the first one has been mailed back to Hagen). I have started turning up the CO2 since last night.

Before I forget, here are the critical tank params:

• 75g
• 78degrees
• 7.3 degassed pH. don't have gH or kH since the last water change, but they have been 4-5 and 5-6 respectively for some time previously
• 4x 52w T5HO 6700k bulbs (though one is burnt out already!, so its really 3)
  plenty of water flow (now, but not a few weeks ago). Currently 9 hours of lights (2 bulbs) with 3 hours of burst
• no air stone running over night (though will turn it back on when adding the fish back into the tank in the coming weeks)
• dosing EI, skipping phosphate over other time to bring the PPMs back closer to a 10:1 nitrate/phosphate ration
• CO2 is being injected with an ext5000 reactor driven by a 528 gph eheim compact+ pump, controlled by a pH controller set for 6.5
• drop checker - blue
• CO2 reading via Hanna Instruments HI3818 CO2 test - ~20ppm

Has anybody tried Hanna Instruments HI3818 Carbon Dioxide Test?
http://www.amazon.com/dp/B0085WWQSO/ref=pe_175190_21431760_M2T1_SC_dp_1

Initial testing this morning indicated that I have ~20ppm. My ph drop via the ph-controlled CO2 setup was set at 0.8 pH drop (7.3 -> 6.5). The drop checker over night turned from dark (almost violet) blue to a slightly lighter blue, but it is definately not green. I didn't get a chance to measure my kH, but it has been around 5 dKH.

Do these results line up? I just turned the CO2 back on so I've not had a chance to see the plant reactions. My fish have been happy with it over night.