The fish have been in the tank for 5 days and I've been doing a 20% water change ever other day. Today my API testing kit arrived and my first test results are:
Ammonia = 0.25ppm
Nitrite = 2ppm
Nitrate = 5ppm
How does this sound?
Pretty bad. You should be up to around 75-90%
daily water changes by now. Fish in cycles are difficult and costly (both in terms of your time and the health of the fish). Ammonia over 0.25 ppm will cause long term harm to your pets, so your aim should be to keep it as close to 0 ppm as possible and to never let it reach 0.25 ppm. Nitrite will (in essence) suffocate your fish to death, so it should also be kept as close to 0 ppm as possible and should never be allowed to reach 0.25 ppm.
Ammonia poisoning will shorten the lifespan of your fish and will make them more susceptible to disease. Which reminds me that it is a good idea to keep medication in the cupboard anyway, as you never know when your fish might catch something. I always keep eSHa 2000 (best anti-microbial medication that I have ever come across: i.e. anti-bacteria, anti-fungal, anti-protozoa) and a small bottle of any anti-whitespot medication (I've got the one by Sera right now).
If I had your tank, I would return all the fish immediately, find a media donor to seed my filter and continue with a fish-less cycle which usually takes less time than a fish-in cycle and little or no lugging about of water. In the mean time, I would research harlequin rasboras and would quickly realise that the tank is too short for them (I can confirm that I have observed
T. heteramorpha growing stunted in a 2 ft tank and growing healthy in a 3 ft tank in identical conditions from the same original stock), so would look at alternative species. I'd find out that it is the length and bottom area (and surface area) of the tank that matter for the stock, not the volume, and 43*26 cm is quite a small space to play around with (as you already know). Suitable species would include larger fish that do not swim around much (like Bettas, so a single male
Betta splendens would work, or a pair of
Betta simplex) or 8-10 small mid-water schoolers of one species (like the smaller microrasboras that mas out at 20-25 mm) would also work. Shrimp have little bio-load impact, so work well in tanks like yours, and you have the total number of fish that would be comfortable in your tank (6* 50 mm mid-water schoolers) spot on too.
and what should I do/expect in the coming weeks?
Lots of lugging about of water, probably 90% water changes 1-2 times per day; testing water every 12 hours, sometimes more often; worrying about the health of your new pets; hours of research about the species, nitrogen cycle, plants and aquaria in general. Fish in cycles usually take 6-8+ weeks, from what I have observed over the years, so you still have a very long way to go before your setup is ready.
If you plan to continue with the fish-in cycle, I extremely strongly recommend that you start using a dechlorinator which claims to "deal" with ammonia and nitrite. Seachem Prime is one that I know does, and apparently Kordon Amquel Plus also does. This is actually a useful thing to have for the first couple of months as a new fishkeeper anyway, or if your tap water contains chloramine.
Cutting down feeding to once every 2-3 days will help you manage the water parameters, and you can increase it back to 1-2 times per day once the filter can support the fish.
Have you tested your tap water parameters yet? They're quite useful to know. The tests to do are ammonia (pre and post dechlorination) and nitrite. Nitrate does not matter as it will probably be 0-5 ppm (which is great).
[…]put in some driftwood and 4 plants.
Are you sure that it's driftwood (which has spent a very long time in water) and not bogwood? Bogwood will leach tannins into the water, which may cause a pH crash that would stall your cycle, which is especially likely if the KH of your water is low (might want to ask your water company about the GH - general hardness - and KH - carbonate hardness - of your water, or get a KH and GH test kit set). What species are the plants?
After 3 days we went with a fish in cycle ( As the aquatics owner said this was the best way ) and added 4 Harlequin Rasboras to start with - we will have six in total in the end and maybe some shimps and/or snails.
This would be the point when I'd stop listening to the shop owner as they seem to care more about their profits than the health of the fish or how much work the cycle will be for you.
Would I be correct in assuming you already know about shooling and shoaling species and importance of numbers, as you plan to go for 6 rasboras? Also, the smallest species that looks like
T. heteramorpha is
T. espei, which max out at 30 mm instead of 50 mm, so would be a much better choice for your tank, although still less than ideal.