All Set Up

The bubbles are out of the water. I presume you used a lot of cold tap water? The colder the water the more dissolved gases it can hold, and vice versa. So as the cold tap water warmed up a lot of the dissolved gases stopped being dissolved and formed the bubbles.



Are you going to plant lots of live plants and use them to take up the ammonia made by the fish, or do you plan on having mainly fake plants?
Testing the water won't show if it is ready for fish because with no fish there is nothing to make the water 'bad'.

If you plan on fake plants you will need to cycle the tank before getting fish. The Stability may help but you can't rely on it to cycle a tank. It may or may not contain the right species of bacteria, and if it has ever been stored incorrectly (eg freezing or baking in a lorry) it probably won't work. It is worth getting some ammonia and adding enough of that to give a reading of 3 ppm, then test after 24 hours. If you then have readings for ammonia and/or nitrite above zero, you need to do a fishless cycle. https://forums.thinkfish.co.uk/fishtank-filtration-and-cycling/fishless-cycling-how-to-do-it/
A 170 litre tank won't hold 170 litres water because most manufacturers include the thickness of the glass and the air space above the water in their quoted volume. I would assume 150 litres. Most ammonia is 9.5% so you would need 4.75 ml ammonia to get 3 ppm. A medicine dosing syringe is ideal for measuring small amounts.

However, if you intend having a lot of live plants, particularly floating plants, they use ammonia as fertiliser. Plant the tank and wait a week or so to make sure they are thriving, then add fish a few at a time, monitoring ammonia and nitrite every day. If you see a reading for either above zero, you'll need to do water changes to get them to zero, and hold off getting any more fish for a while.



You are aware that danios need cooler water than most fish? This will restrict your choice of other fish to those that also need cooler water. http://www.seriouslyfish.com/species/danio-rerio/ We should aim to keep the temp in the middle of the fish's range. In the case of danios this is 22 to 23 ^oC
Danios are fine in hard water, but lots of other fish do not like hard water. Always research fish before buying - Seriously Fish is just about the best site for research.

On the subject of hardness, it would be very helpful if you know just how hard your water is.
If your water company is Wessex water, just type your postcode in here http://www.wessexwater.co.uk/waterquality/
If you are with Bournemouth Water, this is the page https://secure.bournemouthwater.co.uk/postcodesearch/household.aspx
You need the number and the unit as there are half a dozen units they could use and you may need to convert it into one of the two units used in fishkeeping.

Once upon a time, the only way anyone knew how to cycle a tank was by putting fish in there. Of course these fish had to be tough little things to survive all the ammonia and nitrite that built up in the water until enough bacteria grew to eat them both. Zebra danios fitted the bill of being tough, though even their lives would have been shortened by exposure to ammonia & nitrite.
Then it was realised that using ammonia from a bottle instead of ammonia excreted by fish grew the ammonia and nitrite eating bacteria in exactly the same way so fish no longer needed to be subjected to a cycling tank. The trouble is that a lot of books still say to cycle a tank by putting fish in there, as do a number of websites. It can be done this way but it usually involves big daily water changes,while using ammonia from a bottle does not mean many water changes, if any all.

Glad that you're doing a fishless cycle, Pollydoodle. Some local fish/aquatic shops can give terrible advice such as "leave the tank for a week with the filter switched on" and calling that a fishless cycle - that achieves nothing, but a proper fishless cycle as per thread I linked to in one of your first posts will achieve that. [Edited to add: I think I've got you mixed up with someone else - apologies; this is the link I was referring to https://forums.thinkfish.co.uk/fishtank-filtration-and-cycling/fishless-cycling-how-to-do-it/]

A good "rule of thumb" is that fishkeeping is more about water-keeping - and the filter is the fishes' life support machine.