interrupted photoperiod = O2 shortage?

[justDIY]

I was doing some maintanence on my 29 gal yesterday, including a 7 gal water change, trimming a few plants and removing 20 (or so) fry...

I didn't turn my lights 'off' but I did have them off the tank, shining in the side rather than the top

this was for about 6 hours total (a few hours earlier in the day, and a few more at night just before lights-out), as I was watching for more fry to turn up

so, I left the light hood off, and just covered the tank with it's glass cover thingy, planning on waking up the next morn before 'sunrise' (for the tank that is), and checking for fry and putting the hood back on

so I come out at around 7am to check on the fry, and find all my fish gasping for air at the surface (even the tetra that normally stay on the bottom), not to mention a red-tailed shark belly up being nibbled on by fry

wtf I thought, so I get an air pump out, valve, tubing and some stones, and begin heavy aeration of the water ... after I got those going, I checked my ph ... it was around 7, maybe 7.2, down from about 7.8 at the tap, and down from regular of about 7.4 to 7.6

this meant I somehow got a big buildup of co2 in the water? did the plants not getting a full 12 hrs 'sun' not produce enough o2 for themselves and the fish to last the night?

my fishload is on the heavy side (about 25 fish, most less than 1"), but they used to get along fine with hardly any plants (just swords and anacharis), as opposed to the tank that is now heavily planted ... diy co2 has been a fixture for over a year and has never caused this problem

by now most all the fish have returned to normal and are eating again, but it looks like some of my platies are still stressed as they continue to hide in the weeds

I'm leaving aeration going for tonight and will re-evaulate tomorrow.

btw, diy co2 is bubbled into the intake of a powerhead 24x7, regular air is forced into the venturi of the same powerhead via pump during the 'lights out' time from 11pm to 10am... apparently the venturi wasn't working very well at dissolving the air into the water

 

[Electrobes]

wait you have and air and co2 intake for the tank?? In my 10 gallon I just have the Hagen unit (yeast method), but nothing else. But about 2 days ago I took the top off suring the day for about an hour or 2 and the Otos (which are usually sensitive) didn't seem to mind at all... staying at the bottom.

 

[justDIY]

yes, I put a small amount of air into the powerhead during the dark period, to provide a little extra oxygen for the fish because there are so many

plants and fish both always consume oxygen, however, during the chemical process that is photosynthesis, plants combine the carbon from co2 with sugars (in their cells), releasing oxygen ... the oxygen released is usually a lot more than is consumed by the plants and a light bioload of fish .... butttt I think I have over the 'magic' number the tank can support without additional oxygen, so that's why I've been adding a small amount at night, when the plants are no longer producing any

I think my real problem is that I put too much water in (only about 3/8" from the glass top), not leaving enough 'air' on top of the water to allow for gases to exchange

I'll see how they fish are tomorrow and maybe drain down a bit.