Standard Sybas pumpling is steel pipeares mandatory for connecting after the Meter. Hence you see most of the old house use GI, because is the cheapest compare to copper and SS pipe. 

Of course Sybas take action or not when you use difference pipe is another story.

GI also steel ma. GI is already been using for 40yrs or more. Initial is whole house is GI. Than reduce to only main pipe. Than gradually to poly pipe. And sometime I saw ABS or even PVC for the main.

Wonder later will change to flex hose or not.   

But I have been using polyurethane hose with 1 touch fitting for very long. (testing) No problem with the pressure leaking as it can stand for over 10bar for the spec. Very easy work with less tool to fix.

my house now is close to 15 yrs now since completion. my room at the same level as my dad's room does not experience the same problem. the tank has not been washed before. all water tap at the ground floor is fine, just the water to my dad's shower got problem. i'm not sure there's any leak since all other water taps seem fine.

supersound how to open the pipe? it's all within the wall...

PS I have attached photos of the water heater and the tank (located at the window at the top). The tank is pretty high which is sufficient to give enough pressure. Plus my dad's room is nearer to the tank than mine.

from the picture posted i am guessing you meant the hot water flow is slow?
that pipe is copper pipe. if both of you sharing the same water heater and your water pressure is ok, then it'll be pipe block by dirt or stone.
my tap in toilet needs to clean every few months. can find little stones at the wore mesh 

so the tank that you took is not the cold water supply tank la.
the cold water tank would be above the ceiling.
usually the cold water from the tank to mixer is PVC pipe.
guess the mixer blocked. or the shower head

my house now is close to 15 yrs now since completion. my room at the same level as my dad's room does not experience the same problem. the tank has not been washed before. all water tap at the ground floor is fine, just the water to my dad's shower got problem. i'm not sure there's any leak since all other water taps seem fine.

supersound how to open the pipe? it's all within the wall...

PS I have attached photos of the water heater and the tank (located at the window at the top). The tank is pretty high which is sufficient to give enough pressure. Plus my dad's room is nearer to the tank than mine.

just to update, have just replaced the shower mixer, water pressure is still low, can't get enough pressure to use the shower head

is it normal for hot water tap to have slower water flow? seems like my hot one is very slow, both the shower and basin.

the hot water sure lower pressure due to the storage type hot water heater.
There's quite a lot of pressure lost.

How is the water pressure in the kitchen? How about other toilets in the house?
It may be clogged GI Pipes are many forum contributors have mentioned. Have to trace if it is silt from the tank or built up corrosion.
Did you check the supply valve from the tank are fully opened?

if the supply from storage tank to another toilet is ok, then the home water tank supply is probably fine.
On that particular toilet, is the sink ok or not? Just the shower or the whole toilet is low pressure? May be the pipe is block somewhere upstream from the toilet. try to korek2 and see. Else you need to consult a plumber liao.

If all else fail, HACK THE WALL... at least now you have a reason to change the wall tiles. The design is so old

I think the whole toilet has pressure problem including the sink.

the toilet wall has tiles as high as 4-5 metres up till the ceiling. was hoping to solve it the cheaper way 

Have you clean the tank?

And when you ask some plumber cleaning the tank, ask them to clean the internal pipe also. Some plumber have the long wire pipe cleaning with a brush infront. Take out the new mix shower before do the cleaning. Otherwise it block the mixer.

Look for easy solution first before the difficult way.

trying to find someone to clean my tank!

Either change tank, I guess internal has been rusty, not healthy for you, 15 yrs already why still wanna be work.. Nothing can expected

Brand new Joven 25L just Rm699 only