Based on what I've seen with the Hydor heaters I doubt they will affect the flow rate enough to notice, I'm looking at them for my tank as well.The body where the heating element is is a much larger diameter than the hose so it is not restrictive at all. The fittings are the same ID as the rest of the fittings used for the filters so I suppose there is a little more flow resistance added.Just got a couple Eheim 2078s (!) to run the 125 gal I'm setting up and got a few questions...
1) I'm going to use two 200 watt inline Hydor heaters so I'm sure I'll need extra hose, but the Eheim literature says only use Eheim hoses. Is this entirely necessary or should I go ahead and get the Eheim hoses? Followup... Do I need to get hoses from aquarium supply specifically geared for aquarium use or will the stuff from Home Depot do ok?
2) Probably going to paint the plastic input and output tubes black (the rigid parts that actually go in the tank) and I'm wondering about what type of paint to use. Saw in another post someone talking about doing the same thing using spray can Krylon Fusion for plastic, which I already know is a good paint for plastic coating. Will this paint do, or should I look for something that is maybe specifically geared to be less toxic and aquarium safe?
3) The Eheim 2078 listed output is something like 490 gal/hr. How much will these inline Hydor heaters slow that rate?
As long as you get a hose with a thick wall you will be fine. I bought the hardware store hose section last weekend and it is listed on the hose as 5/8"ID and 7/8"OD... not that thin wall stuff. This can't kink easily, which is the only real concern anyway. That size was for my 220 gal/hour filter, so you might be into a 3/4" hose.
Keep in mind that the hose will reach a bit farther, by the length of the heater, as you don't cut a section out of the hose, just cut the hose and put the heater in.
On the paint, I've read the same about that paint but it is mentioned to let it cure for a week before using in the water I think.