Concrete Decor

[Kilrkitty08]

Can I just concrete to make XL fish caves?  I have two big boy leaf fish that love to hang out in slate "boxes"  I'm pretty sure they are 8" X8"X10".   I'm planning on moving them in about 6 months to a 150 and would like a more natural looking Cave area for them and even multistory to move them off the ground some.  Does anyone else have Leaf fish decoration ideas?  The Bala shark in the second picture is why we are moving to a 150gl I was that newbie who went to pet smart years ago and said cute little fish and bought three.  One didn't make it a week the other two well you know the story lol

[Keeg]

Make a small version of what you want to make out of concrete and then test in a separate container to see if it will affect the water parameters. Normally concrete will dramatically drop the ph levels in an aquarium but some people say you have to wait for it to cure for a few weeks and use a mix that is deemed safe for aquariums (ie without polymers, plastics, and junk added). What is a quick and cheap way to make some caves is by using terracotta pots. I would test them like the concrete to be safe. I use little ones for bettas and they love it. Or another idea is to use clay, order some safe clay (I called a few manufacturers to find out). I made a few pieces to my liking and got them fired. 

[Andy's Fish Den]

Concrete decorations can be used, but you have to soak it in water for a long time with multiple water changes while it "cures". Concrete has limestone in it, which will raise the pH easily raising it to over 9.0. When I first started keeping saltwater tanks 10-15 years ago, it was popular to make DIY backgrounds and rock using a concrete based products, and I made a background for my tank, it took a couple months of filling the tank and letting sit for a week, emptying it, and refilling. 

[gardenman]

There's a company called Smooth-On that has a YouTube channel showing how to use their various molding products to make many things, among them being aquarium safe artificial coral. They claim their products are "fish safe" and you could use their products to make pretty much anything you wanted for an aquarium. If you're not overly fixated on appearance, plastic pipe is often used and can come in all kinds of sizes. You can even buy 48" diameter PVC culvert pipe if you've got an exceptionally large fish. Most keepers of really big fish end up using PVC pipe as it's safe, relatively cheap, reusable, cleanable, and readily available. You can get 8", 10", and 12" PVC pipe fairly easily.