Which goes first, inline heater or inline CO2 reactor?

I would say the heater, just so if your CO2 system fails, your heater will still be in water, rather then CO2.

It doesn't matter which one comes first. But you are right that both should go on the outflow part. Actually, only the CO2 needs to be on the outflow pipe because you don't want CO2 going into the filter and being beaten by the impeller because it will just form bubbles and block the filter. The heater doesn't do this so it can go either on the intake or the outflow your choice.

I second that CO2 go into the output, heater doesn't matter.

I suppose someone can argue about when heater breaks, but if that happens, sooner or later everything will die. It is more important to put heater where it is more accessible if it needs to be changed.

Zapins said:

It doesn't matter which one comes first. But you are right that both should go on the outflow part. Actually, only the CO2 needs to be on the outflow pipe because you don't want CO2 going into the filter and being beaten by the impeller because it will just form bubbles and block the filter. The heater doesn't do this so it can go either on the intake or the outflow your choice.

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I have noticed people having their co2 directly into filter inlet hose

I personally think CO2 into the intake is a bad idea.
As Zapins said, impeller will be hitting it and at least causing a noise if not shortening the life of the impeller.
Other reason is that you don't know how CO2 can affect the bacteria living inside the canister.
Chances are its OK, but I prefer to leave them alone.

As do I. Just putting it out there that some people do and say with great results

I have never used an inline CO2, but wouldn't the downward force of the water and the upward force of the gas (lighter then water) cause more friction and get more better CO2 absorption? Does putting the CO2 with the flow of water give you less friction and less CO2 absorption into the water? Do the bubbles really get forced all the way into the canister filter?

Its not that they get forced into the canister filter so much as the impeller knocks them out of solution (like when you shake a soda can all the gas comes out). When the gas comes out of solution it can get trapped in the impeller compartment and cause the pump to stall since it is surrounded by gas rather than water. It also creates a lot of noise and is otherwise annoying to deal with. For those reasons putting the CO2 on the return tube is standard.

Now that I have the Ehiem 2217 I see that it has a 5/8" internal diameter return from the tank, while the output is 1/2" pushing water up into the tank. The Hydor internal heater is 5/8" on both ends and the Rex Grigg CO2 reactor I made is 1/2" on both ends. My plan now is to have the water coming out of the tank go through the heater (with the heater's direction arrow pointing down) into the filter. Then have the filtered water output into the CO2 reactor then back up into the tank? I just need a little reassurance that I am not doing something that will damage or cause any of these components not to work as intended. If anyone has any other ideas I sure would like to see them, Thanks!