Linwood
Aquarium Advice FINatic
Superglue will work even better than silicon as it's almost instant when building, no waiting for it to set as you build.
But that might not be good for the fish. I would go with silicone(aquarium safe of course!).
Actually I think you'll find that it is safer than silicone, which should be completely cured before it is considered safe. It's been used for years for plant attachment to rocks and wood, for example.
It's even used now in surgery to hold skin tissue together as opposed to stitches or staples, and is approved in that mode by the FDA.
I use it to repair myslef on the fly, no better way to re attached a torn finger nail
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I'd avoid Legos because things made in some other countries contain materials that would not be allowed in US manufacturing. Remember the taninted dog food from China???
I'd suggest visiting the hardware store and buying some plastic pvc pipe. You can cut it in section (tunnels) or cut it lengthwise for more open tunnels. If visual appeal is an issue you can be lazy like me and stash it behind driftwood, rocks, plants or just toss the thing in and move on. To make it really nice looking, get a tube of pure silicon caulk from Home Depot, some aquariun gravel and, or sand. Cut the pipe into what ever sections you want or need. Then cover the pipe with silicon and roll it through the gavel or sand. If you use gravel, have some sand handy and once pipe is gravel coated dust the pipe with sand to make sure it is all covered. Decide where you want the hides and push the pipe sections into the substrate so they look like caves. I also use pvc 3" - 4" round slotted drain covers so that shrimp can get away from fish and eachother when they molt. These also work good for baby shrimp especially if you put a dried Oak leaf inside to promote microorganism growth.