DIY water change diffuser
From The Aquarium Wiki
This is a simple idea: pouring water back into the aquarium at the end of a water change/maintenance cycle can mess everything up - plants get uprooted, substrates moved around, etc. etc. The best way to prevent this is to deflect and diffuse the incoming water stream.
Basically, a box that can be hung inside the front of the tank is made, with a bunch of holes drilled in it to allow refill water to spread out gently into the tank.
I would have, or could have, used an old school HOB filter box for this, except my old used 283.9 Litres (75 US G.) tank rim is a full 3.8cm (1.5") wide, so it wouldn't work. Neither would a simple Lee's fish-catching box.
I scrounged around the building for a box that might work, and came up with an old polypropylene tool/gadget box I have owned for years. Cut off the lid, ran it through the dishwasher, rinsed carefully, and bolted on some galvanized steel straps that were also just waiting to be re-used. Then I cut some ugly holes with a spade bit (should have used a hole saw bit to make it pretty) and have used it twice now, to good effect. There is one hole cut in the bottom to make it easier to drain while removing.
Although this should be obvious, once one is ready to pour water back into a tank, this box is hung inside the front of the tank, and the water is poured into it. Instead of a massive stream disrupting the bottom of the tank, water flows out to the left, right, and rear, creating a harmless, gentle, horizontal flow.
The box is deep enough so that in my tank, when doing a roughly 20% water change, the side holes are at least partly submerged.