I'm afraid fish in cycles are very unpredictable in their course so all you can do is follow the advice in the article and keep testing.

The problem is that because you have fish in the tank you cannot allow ammonia and nitrite to rise because they will harm the fish. The beneficial bacteria will grow, but much more slowly than in a fishless cycle. For your situation I suggest reading this article: https://www.fishkeeping.co.uk/articles ... ammonia-nitrite-spike.htm

fcmf is quite correct. This isn't a loach she's a female common bristlenose pleco (Ancistrus cf. cirrhosus)

https://www.ebay.co.uk/itm/Breeding-tr ... 894aab:g:HrMAAOSw7BdfN~HU

Often for the more interesting killis you need to buy and hatch eggs yourself - I just hatched a batch of Nematolebias papilliferus eggs and have 23 fry out of 30 eggs so it can be a very effective way of getting an unusual species at very low cost, or join one of the killi groups and try to source the ones you like from its members.

Aphanias species and some others from the African lakes will be OK in hard water.

While they're not soft water, they won't cope in very hard water, unlike many other rainbows. For Dwarf Neons you'd need gH between 5 and 15 degrees and pH from 6.8 to 7.5

Daisy's rice fish are more soft/medium water so not really suitable. There are a few sellers on eBay offering Oryzias latipes eggs at the moment if you fancy hatching and growing them yourself (I just bought some, coincidentally).

Gold or any of the three other spot variants would work (blue, opaline, and others) although the tank length is a bit short so I'd go for a solo gourami with some other, smaller fish as they are territorial and two or more may just fight all the time in the limited space.

American Flag fish doesn't really do well in water as hard as yours - 20 dGh is about the limit.

Some of the smaller rainbowfishes would work: Celebes rainbowfish - Marosatherina ladigesi, Forktail blue eyes rainbowfish - Pseudomugil furcatus. Also just about room for a pair of Empire gudgeon - Hypseleotris compressa. Another option is Paradise fish - Macropodus opercularis or there are a few different species of ricefish - Oryzias celebensis, Oryzias javanicus, Oryzias dancena, Oryzias latipes.

Looks most like a seed shimp (Ostracoda)

Red poo is usually food-related. Can you check your gH, kH, and pH? If you don't have hardness tests look on your water supplier's site for "Water quality in your area" and enter your postcode for a report on local parameters.