normally an axie tank is cycled with 4ppm to ensure the filtration can cope with all the waste from the number of axolotls, in your case 200 litre is just over 50 US gallon so four axolotls, if you are only keeping one then 1ppm is about right, if you are planning to have more then it is best to cycle at 4ppm.
substrate doesn't slow it down, it speeds up the process as it gives the bacteria addition places to colonise. in a bare bones ie.. axolotl tank it is the filter that is cycled which is why it is so important for it to be big enough.
the nitrogen cycle is waste - ammonia - nitrite - nitrate - water change (plants also consume nitrogen compounds). sometimes you don't see a nitrite increase but if there is nitrate increase then nitrites have been formed just not noticed.
when cycling the water isn't changed unless the nitrates get to 110ppm or higher, this prevents the bacteria from being disturbed whilst a colony is being built up.
there are a few things that can affect the cycling and also the function of a cycled tank... ph temperature oxygenation kh nitrates
the ammonia that is produced is actually two ammonia compounds (TAN.. total ammonia nitrogen, this is what an ammonia test measures), the first is ammonium which is non toxic but also difficult for bacteria to consume, second is free ammonia which is released from ammonium, free ammonia is toxic and is consumed/converted by bacteria. the higher the ph and temperature the more free ammonia is released and the faster the TAN is consumed/converted.
the ph can be so low that free ammonia stops being released from the ammonium and the bacteria starve causing the cycle to stall, even though you get a reading of ammonia.
when nitrates are produced they can affect the ph and cause it to go down, this in turn can then cause cycle stalling, this is why it is important to monitor the ph when cycling and also regularly change the water on a cycled tank. having good kh will buffer and prevent the ph from dropping so that rather than the ph being reduced the kh is instead. having the cycle crash on an established tank due to ph is called old tank syndrome.
as the water gets warmer more free ammonia is released and also warmer water causes the bacteria to reproduce faster.
the bacteria that consume/convert ammonia and nitrites are nitrifying bacteria and use/need oxygen in the water to survive.
sometimes in an aquarium oxygen free areas are formed (under sand, in porous rocks etc or just not enough oxygen in the water) the bacteria that form here are denitrifying bacteria, this type of bacteria used the oxygen in the nitrates reducing nitrates, this produces nitrogen, nitrogen sulphide (eggy smell and poisonous) and also nitrites, (which is why a nitrite spike isn't always a cycle problem it can be a oxygen issue)
it is always better to reduce nitrates with water changes and plants.
as to how to cycle (i'll use the 4ppm). fill tank with water, fit filter, air stones etc.. (better to fit a heater when cycling). add ammonia at 4ppm and leave running for a week.
at the end of the week test the water and top the ammonia back up to 4ppm then retest after 24hrs then top back up, retest after 24hrs top back up, retest after 24hrs etc..
once the ammonia and nitrites are zero after 24hrs top back up to 4ppm for two more days just to be sure, after those two days make sure ammonia/nitrites are zero, do a water change to reduce the nitrates then place axolotl in tank, if the tank isn't going to be occupied straight away top the ammonia up every night to feed the filter bacteria and do regular water changes, the last time to use ammonia is 24hrs prior to adding the axolotl/s (always make sure the ammonia and nitrites are zero).
whilst cycling monitor the ph, if if drops below 7 add enough bicarbonate of soda to bring it up to 7.4 - 7.6. keep an eye on the nitrates but only do a water change if they are 110ppm or higher when cycling or done weekly when cycled, the percentage of water removed is also the percentage of nitrates removed.
if the tank is taking to long (even by axolotl tank standards) check the ph, make sure the water is oxygenated enough, the filter is big enough.
