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Hello fellas,
I'd like to ask your opinion on my problem.
So I got a bonsai as a gift 2 months ago...always wanted one, but never had any before, so it was kinda new to me. Anyhow, it was going great for a while, but I started having issues with it 2-3 weeks ago.
Some mold looking thingy (pic attached) started to appear on its roots and soil.
The main issue is with the falling leaves though...since I got it, 1-2 leaves always fell off per day, which seemed normal, but right now it's looking really bad (pic attached).
And these are the conditions how I've been trying to take care of the bonsai:
- Indoor at 20-22°C, humidity at 50%-70%. The reason I decided to keep the plant indoors, because it's currently winter here, which means ~0°C during the day and it could go down to even -15°C at night.
- It's in a bright window, but no direct sunlight is shining on it (also I have to point out that the radiator is right below the window...so the plant is above the radiator by 40-50cm, I measured the temp though, and it never went higher than 24°)
- Watering: I tried not to overwater it since the beginning, so I never watered it on schedule, instead I checked the soil daily, and when it looked dry, I poured water on it until it flowed through and came out at the bottom of the pot. Since I was watering it with distilled water (so no minerals was added whatsoever), I poured water in a bowl and I also added a solution of fertilizer which contained minterals specificly meant for bonsai, and in every 2-3 weeks or so I sanked the pot under water (water level was at the roots of the plant) and let it sit there for 2-3 minutes.
So I have some general quiestions:
Am I doing things wrong? If so, what should I change?
I'm totally a bonsai noobie, so any help is appreciated, thanks in advance. I also attach some pictures of it to get things more visual.

I’m really not a bonsai person, but just from visual impression, the growing medium looks very dry as in dried out. You may be waiting too long to water. If the medium is dried out, the root ball/medium pulls away from the container and wate will run out without soaking into the interior/supplying the roots. You are probably better off submerging/soaking as you described at each watering. Review how to determine the level of moisture and dryness for this particular plant in case you need a refresher.

Also, the heat is probably drying out the air around it. In my house, the RH starts out around 60% in the fall and steadily decreases due to heated-dry air inside and freeze-dried air outside. We are at about 40% and will see 25% by the end of winter. IMHO max/min thermometer/hygrometer is an essential gardening/horticultural equipment.

I would put a larger tray underneath and set the bonsai on a riser, then fill the tray with water, then the heat will help to evaporate and create local humidity. You can add decorative gravel, etc. if you want, but be sure the bottom of the pot is not sitting in the water. (personally, I think the gravel just traps fallen leaves and junk, get mildew, etc. and is generally more tedious to keep clean /clean up )

First of all, thank you for your answer.
After I've written the post below, I moved the plant to another place, so the heater won't be directly above it. Also sanked the pot under water for a couple of minutesat that time, so it definitely won't be dry. The state looks constant (definitely haven't gotten worse).
I attached a pic of the current state. Always the left side (which got more leaves) of the plant is looking to the window's direction. So the left side is getting more sunshine, and (since it's closer to the window) less heat. The right side (the leafless) of the plant though is getting the heat from the radiator and less light.
So you might be right, and it's the heat which is drying out the plant. I'm gonna try putting a bowl onto the radiator and will use a riser so the bonsai will be much more away from the heater.
Oh and the gravel. The bonsai was a gift from my GF, and the gravel was already in it. I don't really like it either, and Iwill take it out eventually, but I'm gonna wait a little so my girl won't get offended