Recipe for making your own rock

3 scoops oystershell or live rock rubble
one scoop of portland, white if you can find it (swimming pool companies use white, but it will all get Coraline on it eventually.)
2 scoops VERY coarse rocksalt or watersoftener salt (this will leach away to make all the holes)

Mix minimally to a consistency of thick oatmeal.

get a large quantity of sand to use as a mold, lay in a pvc pipe or rope to make holes and remove before completely set. try not to make your mold too intricate, nor too thin. Experience has shown me sets less than 2 inches are just too fragile. Keep it moist during the curing. Let set at least 2 weeks to a month for full curing time. This is a project best done in spring or summer as you can leave the resulting rock to leach out the rock salt in a lake river or puddle for that matter. Point is this is a loooonggg process so you must learn patience Grasshopper. But it is fun and cool and you can make the EXACT shaped piece you desire for your tank.

Sounds fun! I hope we do get to try this some time. I'm adding it to the list of meeting ideas!

I made my "rock" out of the same formula. Menards carries the portland, it's greyish. I does grow coraline. As for the rock salt. Would ice cubes work, for bigger holes, I wonder. I poked holes all over mine with the end of a wooden spoon. I made caves and arches by using a forming around a plastic cup. I also formed them in a dish pan. The backs were flat, but then they faced backwards so who cares. I was able to form corner pieces this way. I alson imbeded dead coral pieces and shells. They do look cool.
I really like the idea of the salt to make them rougher. Cool, thanks

Water softener salt has larger chunks, ice would melt too quickly for the mixture to cure. And it does take a while to cure. I left mine for a couple days and it was not cured completely and some pieces crumbled off.

My entire 300 gallon pond is filled with DIY rock I made using the recipe Nick posted. Takes a long time to CURE after becoming solid too...you have to make sure it's not leaching any longer before you can use it. That said, you also need to make sure your pH test kit is working right, so you don't wait a year on rock that was done 9 months prior!

Ph is the major issue I hear with using DIY rock. Maybe in smaller tanks it doesn't shift as much but what I have read, it can take 3 months to balance out. Though I am not sure what it is that is causing the ph to be an issue, maybe the rock salt? dunno just my .02

After my rock had set, I plunked it into my koi pond. It sat in there for about 4 months, I know not long enough. Then I put it into a 20 gallon and have never had a problem with it. It does grow coraline well, and NOT algae.