Stringy White Stuff Fishless Cycling

The danger with low KH is that cycling creates nitrite and nitrate which are acidic and with little KH there is a danger it can be used up and the pH crash. Below about pH 6.5 the bacteria stop multiplying so the cycle never reaches the end. Keep an eye on the pH level, and if it starts to drop, that's the time to think about doing something.

Sue.

Can't reply just now but will try to do so later. In a nutshell, though, I checked water hardness for your village on SW water supplier website, and my water is quite a bit softer than yours, so, post-cycling, you ought to be absolutely fine with weekly water changes and without the need to resort to adding chemicals (unless keeping hard water fish). Also, test strips results v inaccurate for PH, KH and GH.

I live in a village called Bratton Fleming in north devon on the edge of the moors.

Checking the water supply for your village, I took a postcode of a local business there and its readings are: 23 mg/l Ca; 58 ppm; 4 degrees Clark; 6 degrees French; 3 degrees German.
The important ones for fishkeeping are ppm and German - 58 ppm and 3 respectively. These are more important than PH for fish choices that suit your water.
Looking at the most up-to-date source for different species' requirements that JPC gave you the link for, the Agassiz cichlids would be fine in your water (0-179 ppm is their required range) as would the cardinals (18-215 ppm).  However, your water is too soft for galaxy rasboras / celestial pearl danios (90-268 ppm) and dwarf rainbows (5-15).  Your options, therefore, would be to consider fish whose hardness requirements do fall within those of your tap water, or to consider hardening it post-cycle to ensure the needs of all of these fish.  JPC and others ought to be able to help with the latter if that's the route you prefer to go down.

As for the test strips, they are very unreliable for PH, KH and GH.  I have alkaline PH due to the water supplier adding orthophosphates but the test strips show it up as 6.8 rather than 7.5+.  The test strips show a much harder KH and GH than my actual <2 GH or 34 ppm. However, I still use them as I find them good for the chlorine, and quick/easy nitrate and nitrite readings (as I find liquid-based test kits very difficult to decipher).

Hope that helps.  Good luck with the cycle - hopefully the bicarbonate of soda ought to help considerably.

Some suggestions, as I've "been there" myself with my similar/softer water and these fish are on my radar:
* rasboras/bororas: harlequin rasboras, espei rasboras, hengeli rasboras, neon green rasboras, or a whole host of lovely tiny bororas (eg chilli, maculatus);
* tetras: ember, ruby (not so common), cardinal (which you're planning), green neon, neon, lemon (meant to brighten up once settled in tank), rummynose, flame, serpae - would avoid this one as nippy;
* coral red pencilfish;
* honey gourami.
Hope this helps.
NB. X-ray tetra is lovely too - fins are colourful and the silver body actually contrasts nicely if you have other fish with colourful bodies.

As for the test strips, they are very unreliable for PH, KH and GH.  I have alkaline PH due to the water supplier adding orthophosphates but the test strips show it up as 6.8 rather than 7.5+.  The test strips show a much harder KH and GH than my actual <2 GH or 34 ppm.