You need to stop adding new fish until you get things sorted out. What you will see below pertains to the tank where you tested for .50 ammonia recently. Doing all the other things you have since then in terms of adding fish should be undone and you she be back to the point you were when you started this thread.
Before reading further I want to ake it clear the the best solution to most fish in cycling problems that do not involve an impossible level of overstocking is to add bacteria to the tank, this is the safest and quickest fix possible, this can be done using Dr Tim's One and Only or Tetra's Safe Start, both viable bottled bacteria.The other choice is to get more cycled media from anybody you can find willing to donate some. In the states there are people who sell cycled media, I don't know if this is done in the UK. That said.....
OK- great with the numbers, we can make some progress. The first step is for you to understand what makes ammonia dangerous. There are two forms of ammonia when it is in water. One is aamonium, aka NH$, and the other it the actual ammonia gas, NH3. NH3 is highly toxic to fish and at a level of .05 ppm is harming things for sure. NH4 on the other hand is way less harmful and fish can tolerate much higher levels and for longer. However, this does not mean NH4 is always harmless no mate the level or how longer the exposure. Elevated NH4 levels can burn fish. For most fish, it ammonia were 100% NH4 they can usually handle up to about 2 ppm for a bit of time and some fish can take much higher and/or for longer before its an issue. I have seen research studies where total ammonia levels reach double digits and it did not kill the fish even when exposed for days. Please, do not try this at home.
Now, for the next part of this. When we test for ammonia out kits measure the combined total of NH3 and NH4. So how can we know how much of what we tested is in each form? The answer depends upon the pH and temperature of the tank. These two factors determine what the levels are for any given tank and test results. There is a complex formulas for determining the answer. But there are also pre-made charts or calculators for this which makes it possible for mathematically challenged folks like me to get the answer. Here is a good one, note, it works for both fresh and salt water and all in between. For those with fw tanks the salinity number to enter is 0.
http
/www.hamzasreef.com/Contents/Calculators/FreeAmmonia.php
The only choice you need to make is the very first box which should be set to NH (NH3 + NH4). This matches your test kit results. The answer we want is the NH3 Concentration. When I run the pH and temperature numbers you supplied, your NH3 level was .0036 ppm. Your fish could likely live their entire lives in that level. While different sources peg the danger level for NH3 at different numbers, most agree .05 ppm is the level where harm starts being done. The other thing there seems to be agreement on is that at under .02 ppm, virtually nothing in a tank is at risk. So from that point of view, your NH3 ammonia would need to rise by 550% (5.5 times) before it was even approaching a minimal level of concern and it would have to increase 1,388% (almost 14 times) to be at a level where for sure it is too high.
So what does all this mean? It means you should not be thinking about changing water until your test results approach between 2 and 3 ppm for ammonia at a pH of 7.0 and a temp of 28C. In fact, it takes a reading of 7.0 ppm on your test kit before NH3 hits that .05 ppm level in your tank. However, the ability of ammonia as NH4 to cause burns would argue against allowing a 7.0 reading to be allowed to go with out action to lower it.
The point here is to get the tank in question cycled for sure, you need to allow your ammonia levels to rise a lot higher than you have been so far. The higher the better as long as the NH3 stays under .05 ppm using the calculator linked above. Of course, there are no hard and fast rules because different fish are differently sensitive to ammonia (and nitrite). So we also need to monitor the fish. If they are showing signs of distress or are behaving abnormally, this trumps all the numbers and test and one needs to act.
Now, this only covers the ammonia part of things. Nitrite is also toxic to fish. However, the way it harms fish can be counteracted over the short term by adding chloride to the water. Common salt is sodium chloride and chloride is about 2/3 of the mix. So plain old salt can be used shot term to prevent nitrite poisoning. This works to block the effect of nitrite inside the fish but does not do anything to prevent the bacteria from getting the full benefit. But I wont add to what you need to learn about this side of things until you start to have nitrite readings. And let me know in a PM to be sure if you want my suggestion on how to proceed from that point.
I will close by saying I hope the above information has been daunting and seems like it is much more complicated than you like. You need to be more of a chemist than most really care to become. And that is one of the best reasons to do a fishless cycle and not a fish in one. Your best course of action here is to rehome the fish and complete a fishless cycle. Afterwords, you can take the fish back. Equally good, if you want to leave the fish in, would be adding bacteria, enough to cycle the tank in a matter of days rather than weeks. This matters to the fish. Many stores will take back fish for store credit if you explain you don't want a refund, just the chance to complete the fishless cycle and that you will be back with the credit to use there in a few weeks. Note: there is a level of stocking in any given tank which will make it impossible to get that tank cycled with fish in no matter what you do.
