I like rearranging my aquarium about once a month or two and I much rather make my own decorations than buy those expensive cookie cutter ones at the store. And I like to have unique pieces that others dont have
Step 1: Pvc Pipe
I made this out of pvc pipe. I bought a 3 inch curved piece for the bottom and two 2inch pieces that I connected for the top. I cut the large end (for connecting to another piece) of the 3inch pipe off, as well as one of the 2 inch ones. I used these to make a side little mini tunnel.
Step 2: Cutting Holes
I cut random holes in the pipe with a jig saw. It's pretty messy and leaves extremely jagged edges so I sanded them smooth with a dremel. I decided to leave the smaller top piece without holes so that it could be a private little tunnel for my shrimp to chill in and conduct secret meetings and trash talk the other fish.
Step 3: Spray Foam
I got this black pond foam from lowes which is used for sealing koi ponds and whatnot, and is said to be fish safe. Anyway I sprayed the top and bottom pieces with it and after it dried I situated them how I wanted (small tunnel on top) and sprayed where they touched with a thick layer that acted as an adhesive holding them together.
Side note : this foam is extremely sticky, almost like tar. And it's hard to get off pretty much anything, I used a plastic spoon to spread it over the pvc pipe and I went through a handful of them because once the foam hardens on the spoon ( a few minutes) it is hard to keep using. And I wore gloves (after learning the hard way). Also this stuff, like shaving cream, expands a lot. So account for expansion when using it.
Step 4: Paint
Krylon fusion spray paint (in my experience) is aquarium safe and completely inert once fully dry. I sprayed a little pattern on it and let it dry for a week (I've let other things dry only 24 hours and put them in the tank but I just was real busy) and then put it in the tank.
Step 5: Let the Fish Explore Their New Tunnel
My shark swims in and out of the top tunnel and had claimed it his own pretty much, the bottom part is free territory but he also ventures down there pretty often but my cory cats like it too. My betta refuses to acknowledge it's even in the tank cause he's a little punk. But that's fine cause I built a little watch tower for him that he likes to chill in and overlook the aquarium.
Important note with this foam: it is extremely, extremely buoyant. Meaning it was a pain to get it to stay down. So I filled the bottom with those cheap glass rocks you get at any generic craft store or even Walmart. (though Walmart should never get your business cause it's the scourge of the earth) and that was still not enough, I had to put a big cheesy fake yellow diamond on the top to help force it down. So I didn't plan ahead on that part.
Step 6: Other Decor I Made
I made these out of sculpey. It's pretty easy to work with and you just have to bake it in your oven at 275 for 15 min and it becomes hard and aquarium safe and I sprayed them with krylon fusion paints as well. I make all of my own decorations except for the plants obviously. It's nice to have your own work in the tank for the fish to crap all over :)