Your tank is big and cycled so when you add more fish is not really a problem as long as you can supply them enough oxygen.

If you can have CO2 in your tank while the light is on, your plant would establish the root fast using CO2 instead of O2 and produce lots of O2. Once they're rooted, they won't use much O2 while light's off like they're now anymore.

My 200l with 20+ inhabitants don't need bubbles even when light's out. Just continuously and slowly adding CO2. For my other 60l with 7 fish, I use CO2 tablets (2 tablets/week) and it works pretty fine

I do like to have the bubbles on anyway, I have 2 underwater LED lights (alternating 3 colours) and the bubbles come from them , as the bubbles rise, they are illuminated by the LEDs. They just get in the way a bit when I'm looking in from the top which is why I had to switch them off 

How is your filter set up? Theoretically, as long as there's enough surface agitation (which is what you're getting from the air pump- very little oxygen is dissolved as the bubbles rise) there should be enough oxygen being dissolved in the water. As long as you're getting enough surface agitation from the filter outlet I'm not convinced air-pumps are necessary.

I have quite a few plants (although without seeing your tank I couldn't say if it was more or less densely planted than yours), and haven't seen the fish gasping at the surface other than after a power cut, when there was no surface movement for a several hours.

The fiter comes back in to the tank a fair way down, 6 inches or so, no surface movement at all.

I think Noodles probably had a point with the plants establishing, roughly 25-30 of them planted in a little over a week. Starting to look better, just need them to start growing 

I'd sy the number of plants is a little less than your 240L but it's hard to tell when mine don't reach half way up the tank yet 

What Dave said makes good sense. I notice that all my tanks has water surface movement. You should also try to change your filter or bring it closer to the water surface

Quote:

As Dave said, you'd be best positioning the return so that it causes some surface movement. Sorry I didn't mention it and just assumed it was set up to break the surface . If you have a spray bar you can position it in such a way that is doesn't cause a massive disturbance if you prefer, just so long as the suface of the water is being broken.

While the plants are establishing themselves they will be more metabolically active and without any surface movement from the filter return the CO2 produced by the plants and fish would not disperse readily. This was OK with the bubbler running and only a few fish, but when it was accidentally left off, CO2 built up once the lights went out. As you add more stock, this will become quite an issue even when the lights are on as more fish = more cCO2 and the small disturbance cause by the bubbler won't be enough to achieve proper gas exchange.

When you say spray bar, do you mean the tube with little holes in?

I have that facing up but it doesn't seem to move the surface, the fish like swimming down into the flow.

Had a look at the inlet and there doesn't seem to be any way I can shorten the pipework without sawing some off. I have angled it up as much as I could without it coming apart though 

That's the spraybar. By "inlet" I take it you actually meant outlet? Most filters tend to have the inlet quite low in the tank, to get a current sucking gunk into the filter where it can be broken down, and the outlet up at the top to give you surface movement.

What filter do you have?

Would have to check what make the filter is tonight, another 9 hours before I'm home from work. Yep, inlet and outlet were the wrong way round. It's the pipework with the spraybars I have angled upwards.

I have one filter using the spraybar facing directly upwards and the other one comes out the end of the pipe vertically.