[Tank Temperature]

27 deg C is ok ...

[resun cl650 driven by which pump better ?]

Any pump that is capable of delivering between 1500 to 2500 L/H flowrate.

[sump, refugium...]

For a normal horizontal sump, the refugium should ideally be the last stage and taking this into consideration then it should be the bottom compartment.

[Need recommendation on Pump & Skimmer]

Basically, you just need to cater a pump which can provide total water movement about 8 to 14 times the tank volume.

For skimmer, there are many options available in the market and depending on your preference whether to opt for needle or pin wheel impeller or Beckett.

Sorry can't advice here as it depends on how much do you want to spend but I can't stress enough that you must not skimp on this piece of equipment.

So have a good budget for this fella because a less powerful skimmer would affects your water quality.

[CO2 with solenoid USES??]

What I meant is that when the solenoid got problem then can bypass the pH controller. Afterall pH controller don't come cheap leh.

Most CR problems iniitially are always associate with tuning and solenoid if it is previously use in a planted tank environment

[CO2 with solenoid USES??]

Otherwise don't install the solenoid and you can bypass having a pH controller in your CR setup.