Some questions...

[domzzz]

Tank Dimensions (LxWxH): 39 x 24 x 23cm 20.9l or 5.5 gal
Lighting Intensity(No of Watts) : 22W pl
Type of Lighting (FL/PL/MH) :PL
CO2 Injection Rate (bps) : 8-9 bps
Type of CO2 (DIY/Liquid/Tank) : DIY
Substrate Used :
Liquid Fertilizers Used : Sera Florena
Frequency of fertilization :
Tank Temperature : 26 - 28deg


Chemical Properties (Fill what you can)
---------------------------------------

Carbonate Hardness (kh): ~9
Total Hardness (gH):
PH : 7
NH4 (ppm):
NO2 (ppm):
NO3 (ppm):
PO4 (ppm):
Fe (ppm):

Bioload (Your Fish and Plants)
------------------------------
2 oto, 2 sae, 4yamatoes, 8 neon tetra

Describe your problem :

Is the KH at 9 too high for my tank.
Cos i heard from the uncle that sell me my stuff that KH should be around 5

If it's too high, how to lower?
Does water change help?

There are some sticky stuff at the airstone, i think it's yeast from the DIY, Is it harmful to the fish?
If yes, how to prevent this problem?

Thanks
Domzzz

[hoppinghippo]

kh 9 is a little on the high side, maybe 5,6 would be better. check the kh of your tap water at home, some places have high kh tap water, if your tap water is low in kh than yes water change will help lower your kh.

your co2 rate is very high yet your ph is only 7. I think you're losing a lot of co2. try using a reactor if you are using an external cannister filter. I think a bps of maybe 1-2 is ok for a tank of your size. my 2ft 70L tank uses 1.5bps only! try to aim for kh6 and ph 6.8, this will give you 30mg/L of co2.

you should connect a bubble counter (if you haven't oredi) btwn your co2 and tank. else the yeast sugar mix wil surely go into your tank sooner or later! donno if its harmful to fishes, maybe you'll get drunk fishes!

[David]

Actually, KH of 9 is still alright, the plants will be fine. Its the fish that may react. From the tank population I see, they can deal with 9. But if you insist on lowering your KH, do not exceed 1KH point within 24hrs.