Two days ago I started injecting CO2 using the Hagen natural plant system.
To avoid loss of CO2 I turned off the air driven under gravel filters.

Yikes! Make sure you monitor your water parameters. Check for ammonia and
nitrite. You just killed off your biological filter. It might be okay, if you
have another filter on the tank, and lots of healthy plants. Still...keep an
eye on it.

Is this gasping or some other sign of low oxygen?

It might be. I noticed oxygen was low in my tank when I saw all the fish in
the top half of the tank in the morning. They weren't actually at the surface,
they weren't gasping...but it was unusual, that they were all in the top half
of the tank. Putting in a powerhead fixed the problem.

However, I would measure for ammonia and nitrite before assuming it's low
oxygen.

My PH has dropped from off the scale (I am using the test kit for my pool,
Max is 8.0) to 8.0, so I believe the CO2 injection is working. I can see
the bubbles get smaller as they travel though the diffuser, which also tells
me the CO2 is dissolving in the water. I don't know actual levels.

It would be good to know just how much CO2 is in the water. It is possible to
have too much.

What is a good test
kit For Kh, PH and CO2?

I use Aquarium Pharmaceuticals. Judging from what has been posted here in the
past, the Red Sea test kits are worse than useless. I don't buy them, even if
they are the only ones in the store.

If your water is very alkaline, look for a high-range or wide-range pH test
kit. I think my wide-range pH test kit is by Tetra, and it works very well.


Leigh

http://www.fortunecity.com/lavender/halloween/881/