happen to me before, small worm wriggle in the tank. lucky i got my guppy inside the tank. suspect came from leftover cucumber that i feed to shrimp

yes it depends... mine was quite problematic, d layer on water surface,  i swear i can even pick up the layer with lidi... some say its because of OD antichlorine, but i dosed ngam2... some cfu here suggest black molly to fix, but my tank is quite crowded adi... so last choice, use surface skimmer... ugly piece of equipment (planning to cover it with moss later...lol...) but, does its job perfectly

I really wished I can handle a HC soon Do you think I can try HC in my current 1ft tank with 13w light, non co2, no soil LOL?

try with glosso 1st. once you managed to grow the glosso, then you can try HC.
HC is has higher light demand compare to glosso.

Okay, I will try glosso Since it is also a foreground plant!

When putting ADA soil into a new tank, is it necessary to put 8cm of thickness throughout the tank? Can I just pour ADA soil where I will put my plants?

3" of ADA soil??
 
i thought 2" is more than enough.. unless you want to create slope. but then you can use rocks to save on soil..

and if you have an idea how to scape your tank, you can just put the soil at the specific place where you want to plant.
but then, how you want to fill the gap between??

Erm, please answer this question; the soil is only needed where plants root underneath it right? Otherwise soil are not needed.

Someone told me 8cm of soil Then the other day I was trying to put 8cm of small gravels on my 1ft, only to realize it was a massive amount

Erm, please answer this question; the soil is only needed where plants root underneath it right? Otherwise soil are not needed.

ya ya...your plant need nutrient, which is why we need aquasoil, not just plain sand, which contain a little amount (maybe not) of nutrient that plant need. you can save soil by putting divider (such as rock / wood) form sand to seperate them from mixing together. dont put hyper active fish such as GAE, they will mix it together. 
anyone want 3 inch SAE? got 2 of it. free of charge  

Okay Get to learn a new knowledge on the soil for the plants, thanks!

What's SAE?

catch those diseased, separate to other aquarium, n medicate... n do massive waterchange i guess... thats what i do...