Possible Changes To The Tank - Thoughts Welcome

Both of these species are from dimly lit environments, so I would suggest a darker substrate, a dimmer light than a standard tube, and more plant cover. Also a few more fish might help - either more of the same or another species or two (dither fish need dither fish too )

Is the water level normally as low as it is in your pictures? This could be causing refraction of light down the glass panes making it difficult for the fish to see through them, which could be adding to their nervousness.

Thanks for the replies so far.

I ought to have clarified for contextual purposes that the fish seem very content in their dim environment, with the exception of two situations - the weekly or twice-weekly water changes and the tank light ever being switched on which is very rarely done for the aforementioned reasons. The tetras' dorsal fins are strikingly bold and their tails very bright red, and the harlequins' patterning is similarly bold with the pink/orange shimmer. The behaviour is normally calm and relaxed - swimming (not usually in shoals, though), rest at the various "staging posts" (interspersed with occasional spats/bickering), and some long episodes of mating among the tetras for several mornings in a row every now and again. [During and following water changes, though, the tetras' tails go very pale, and all 12 fish shoal together. If the tank light is ever switched on, which is very rarely such as to take photographs, then, despite forewarning from sitting by the tank and opening the lid and then gradually closing it so that the light strength is gradual, there is complete panic and darting around in alarm and cowering under the silk plants, and which doesn't seem to cease with time.]

@hampalong:
*Yes, the water level unfortunately can't go any higher if I continue to use the filter which came with the tank. It has a waterfall effect and should be an inch above the waterline - unfortunately, it's not possible for it to be pushed up any higher in the tank than it already is. [I also have another filter in there which provides water surface movement.] It might be that I have to consider replacing the filter in order to increase the water level - the refraction (most likely when the light is on, given that they seem quite relaxed otherwise) isn't something I had thought of before.
*I'm pretty much stocked to full capacity, judging by the Community Creator, so I think any additions would probably have to be of the same species although open to further thoughts on this from anyone.

@Richard W:
*The back and right-hand-side are covered in dark blue aquarium background paper; it had originally been black but I changed this a few months ago; due to the location of the tank, and to be able to view it from the sofa, the left-hand-side hasn't been covered, and the fish seem to like this particular location when the table lamp adjacent to the tank is switched on (despite their extreme dislike to the lighting in the lid of the tank).

With real, live plants, I'm going to have to get some form of lighting, certainly a lot dimmer than this if the fish are to tolerate it  https://fishkeeper.co.uk/product/fluval-aqua-glo-t8-fluorescent-aquarium-bulb-40w-107-cm . Again, any suggestions welcome.

I'll take a few more photos if I can, which might help others to suggest further thoughts; thanks.

The bogwood, with Java Fern, Anubias, Java Moss etc is a good idea. Some of my tanks have the top completely covered with Amazon Frogbit (helped, unfortunately by a fair amount of duckweed) which dramatically cuts down the light level. Having said that, the filter you describe doesn't sound very compatible with these.

If you want a dimmer light, I'd suggest trying to get LED lights which can be controlled with a dimmer. That way you could start very low and slowly increase the light levels.

I have 20 similar-sized fish in very slightly larger tanks (X-rays and Glowlights in one, Lemon and Flame tetras in another). They've been in for 2 years with normal to low levels of filtration with no problem, so you might consider adding still more fish.

Thanks, folks - some really helpful ideas here.  I'll take a deep breath and take the long-overdue plunge into "real aquatic plantkeeping"...

Would LED lights be suitable/attachable for the inside of this lid, do you reckon?  (It's slightly curved and tends to get very wet with condensation.)

I have anubias
I agree with richard

My plants are all java fern/anubias/bolbitis on wood too. I even grew java fern in the old betta's tank which did not have a light, though it did have a perspex cover which let light through.

Thanks, folks.  I thought the tank was plant-heavy and why the fish enjoy spending so much time in the part of the tank without the plants ("the clearing" as I call it), but this has all been quite enlightening.

The tank already has bogwood in it but I bought a second piece today which I've been soaking in a bucket which I've been topping up with boiling water, to try to help it to sink and hopefully minimise the slimy/fluffy stuff which I had for the first 8 weeks or so on the first piece. For how long ought I to soak it for?

As mentioned in the "quarantining plants" post, I've bought anubias which I've been "preparing".

Once the bogwood has been sufficiently soaked, I'm going to attach the anubias to the bogwood in various places - hopefully it will grow onto the bogwood during its time in the QT, and the piece of bogwood with its thriving (I wish!) anubias can all can be moved into the QT in a few weeks' time.

Update a little later: been browsing through the plant section on this forum, and noticed in the following thread http://www.thinkfish.co.uk/forums/fish-tank-plant-advice/lighting-for-plant-growth/ about the potential effects of the lack of calcium on plants in a soft-water aquarium, so I'm going to add a lump of limestock rock to the QT.

That's exactly the same tank I have, I now have a sponge filter in it as its my shrimpery and pygmy cory tank.

Yes, I remember reading your post about the Marina i110 filter and how you changed its filter cartridge - I followed that advice (and Sue's) for converting its media to a sponge one. What filter do you now have in there?

Do you mean the filter in this thread?

Thanks, both.

I'm not sure that I'm the right person to respond to this, but I would always consider an extra tank.