Thank you for answering all those questions it certainly helps.
It should all be washed once a month using the old tank water.
Carbon is an excellent product for collecting chemicals in a tank. BUT and a huge but it not only collects them it also stores them and when its full it slowly releases a cocktail back into the tank.
It is perfect to use after medications then only for 2-3 days then its a TOSS OUT.
It is not necessary in a tank unless you have just finished using medications.
It also absorbs plant additives.
Now we have several concerns here.
It looks like the tank is cycling and does not require a water change during that process unless using special plant media. Then a minimum of 30% weekly.
I prefer to do 2 30% weekly.
Because you are syphoning you are disturbing the good bacteria in the substrate while it is still Cycling.
That should not be a problem But the carbon could be removing it.
It is obviously still cycling
Nitrite should be "0"
Nitrate Should be below "10"
Have you run that program its most important?
If not or you have please post all the results.
I can see some problems there.
This will also help you reason being your fish might not be compatible or the tank is over loaded
Warning signs of "Cycling"
I hate to say it but what a mix who advised you to use all that.
In 45+years of fish keeping I have never heard or seen such a mix.
Sorry I would toss it all out and replace it with a good quality aquarium grade natural river stones 1-3mm at least 100mm deep this is best for healthy growing plants.
Creek rocks unless you know exactly what they are they could be causing problems, I did say 'Could"
Plants real that is excellent.
BUT with that substrate dont expect them all to grow.
I also notice you are not feeding them.
You are also not using a quality Bio-Starter like Seachem Stability.
For a beginner the Aquascape is excellent.
I can see many problems its the substrate depth and placement of a few plants because of what the could grow too if given the correct TLC
I think I have covered every thing, sorry if I seemed harsh but I would rather tell you the truth than sit back and not reply to you