You're right that some/most of the canister filters in the size range you're talking about have 1/2" inch hose fittings. The choice of hose size is mostly based on the required flow rate, so basically any canister filter whose flow rating is on the order of 200-350gph will use 1/2" inch hose.
You're also right that you can shop around and find 'hose barb' to threaded pipe connectors and threaded couplings to build 'proper' plumbing adapters to mate 1/2" and 5/8" hose, but that this will add up to an extra 10 bucks or so. Now I won't officially recommend this to anybody else, but when I need to mate hoses which are only 1/8" different in ID
, I just silicone tape the smaller fitting to build up the diameter to match the larger ID
hose, slide the hose over the tape, and then secure it with a hose clamp. After all, where canister filters are concerned we're talking about stopping leaks with water pressures of 2-3 psi, not the 30-60 psi applied to a garden hose by city water.
Also, while this probably isn't a big deal, the ETH300 heater module is probably expecting a flow rate of 400-500gph to carry away the 300 watts worth of heat it provides, which is the reason that it was provided with 5/8" hose connectors. I'm sure that the Hydor heating element won't be harmed by a flow rate which is significantly lower than it was designed to operate with. However, you may find a significant temperature difference between the water in the heater module and the water in your tank. Actually, if it were possible, you'd be better off in several ways by exchanging your ETH300 for an ETH200 with 1/2" hose fittings.