New Tank Syndrome?

Hi all,

We recently finally completed decorating our front room and set up our brand new Aqua Oak 220 litre tank. We completed the cycling process (allowed 2 weeks) and tested the water before introducing 6 Congo Tetras. We allowed another week to pass and added a further 4 Congo Tetras so that we now have a shoal of 10. We also purchased introduced 2 Pearl Gouramis into the tank the following day but they began to behave very oddly either sitting on the bottom or frantically darting around hitting the sides of the tank. I checked the following morning (today) but no change so I thought I would leave them for the day to see how they got on.

I arrived home this evening and still no change and it looked like they were both at the brink of death. In fact I was so concerned I decided to move both of them into our 60 litre tank which we have had for quite a while and is thriving. There was almost an immediate difference to the Gouramis behaviour with both of them happily swimming around and they even ate some flake.

I can only assume that although the water tests were all showing perfect, the chemistry isn't quite established yet. I'm thinking of leaving the Gouramis in the smaller tank for another few weeks until the bacteria levels are more established in our main tank. The Congo Tetras appear fine although are still quite nervous and frequently hide behind some rock/tall grass in the tank.

Has anyone got any other ideas or suggestions.

Thanks in anticipation 

@animal_freeride With just three weeks of cycle (was it the full fishless method described by Sue in Filtration and Cycling?) it could be the cycle has not finished yet, but the bio load you have in the tank is low and I am not surprised that the tests results are clear and good.
If the water tests are clean then this shouldn't affect the fish as you describe. How are the two tanks decorated and lit, could it be fear that has driven the behavior change?

I think the crux of the problem is that the tank didn't go through a proper fish-less cycle which is actually the preparation of the filter to be able to process the fishes' waste; preparing it involves dosing the tank with ammonia (which simulates the fishes' waste) for several weeks until it reaches the stage where it is able to effectively convert the ammonia to nitrite and then nitrates - https://forums.thinkfish.co.uk/fishtank-filtration-and-cycling/fishless-cycling-how-to-do-it/. If you haven't done this, and instead just let the tank sit empty (+/- decor) with the filter running, then this is just going to give you more-or-less the same water quality results (for ammonia, nitrite, nitrates) as you would get from tap water although this may to start increase now that fish are present and the fish-in cycle is beginning.

As you now have fish, though, do not add an ammonia source to the tank that the fish are in as the fish will be producing this - it's best to follow the method for a fish-in cycle https://forums.thinkfish.co.uk/fishtank-filtration-and-cycling/fish-in-cycling-with-fish-how-to-do-it/ and https://forums.thinkfish.co.uk/fishtank-filtration-and-cycling/seeding-a-fish-in-cycle/ which you're inadvertently undertaking.

Keep monitoring water quality levels in the established tank where the gouramis have been moved to - that does sound like a sensible move for them to be in there for now, in light of the immediate improvement in their behaviour, until the bacteria levels establish themselves in the 220-litre tank. Decor, lighting (is it a lot brighter than in the 60-litre tank?), etc, as AtM suggests, are worth addressing in the 220-litre tank - and your suggestion of the addition of more plants and rocks sounds sensible.

[Edited slightly to update.]

I would highly recommend immediately seeding the filter in the new tank with a small portion of media from the old tank (not too much as you just added two large fish to it) this will help it support the fish in there currently which I believe are not showing the same signs as the gourami only because they are more robust fish.

If a portion of used media from an established tank is placed inside the new filter at the beginning of the direction of water flow ('seeding' the filter), the process of cycling a tank can be significantly quicker, easier on the fishkeeper (by reducing the need for water changes) and (more importantly!) less impactful for the fish.   This can be achieved by removing up to a third of the media from an existing tank when adding a tank to your collection or when upgrading and shutting down an old tank by moving across all filter media into the new filter for example.

Also, do a water change asap on the new tank!!!