I'm going to warn you ahead of time, there is going to be a lot of members flaming you for putting fish with the potential to grow to a massive size in a small tank. The 100g will work for 3-4 months. You'll see soon that these fish grow very fast.
Now back on topic, did you cycle your tank? Also, do you own a water test kit that you can see what your water parameters are?
rtc's price here in the philippines is only $8 it only 3 inch,, my australian aro's price is only $17 is it cheap?., rtc here in the philippines is not popular., unlike the aro's here., evry house here they have their own aros.. just wanna share.. ;p
don't "upgrade and upgrade"...unless you want to keep buying tanks odf hundreds of gallons each at yearly increments. Just get the huge tank that will house the fish for life now.