You have approximately 7" depth to work with which will accomdate any 25 watt and a lot of 50 watt heaters. There needs to be a baffle directly against the impeller platform area and make it so that the bottom of the baffle is about 1/2 to 3/4" above the very bottom of the hob lowest portioon of the floor directly adjacent to the pumps raised platform area. Its also good to have the area above the platform area with holes so water also flows thorugh it as well. The side of the original filter miedia basket cut to shape and size works perfect for this baffle. Leave a space between this baffle and the next which needs to be a solid piece at least 3" high to keep sand etc from getting drawn back in and around the first chamber. It needs to be attached from the bottom and up the sides. Leave about a 3/4" space and install yet another baffle inline and on top of this lower 3" baffle. The space between the top of the 3" and the bottom of the upper is suffiicient to channel water out, after it flows thorugh any media or carbon etc you put in that first chamber. I use a coarse sponge cut from included white sponge in the shape of an inverted letter L to act like a mechanical filter, yet provide space for adding carbon etc and placement of heater. If current is still to much into the fuge chamber, another short small baffle baffle can be added from the fuges water level down to about 3/4" off the fuge bottom, and spaced about 1/4 to 1/2" from the last split baffle. Even if filter sponge clogs up, the space around the intake tube is sufficient and designed to allow the water to flow out around the inflow tube, and it can also flow over the baffle tops and sponge into the main fuge area......For all the baffles adn the water spillway guard they can all be cut from the hob filters media basket that is packed with the pump.......only thing you should have to buy extra is marine goop, as its all makeable from the media basket and included sponge etc.