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So I had an idea.
A kind of mad idea... but those are the best ones, amirite? RIGHT?

Let me go on a small tangent to explain...
My GF decided to gift me with a few fish while I was away for a vacation. Unfortunately, these fish are about to grow way to large for my current tank, so a plan was made to get a new, larger, fancier, better tank with all the things I learned since I got into watery fishy stuff. the new tank was ordered an is on it's way, but being the perfectionist I am I kind of had the idea of a smaller second tank to "outsource" the fish that wouldn't want to be in the big tank.
And then it struck my like intuition from the Emperor's own tarot.

I could actually produce a 40k Diorama.
Under water.
With Space Marines or Skitarii in a besieged position, looking and firing upward since they're under attack by some weird fish looking flying xeno species!
Heck, they might even be assaulted time to time from small, reddish looking crab creatures looking for algae (or tearing their head off, depending on your minds eye)!

But before I can do that (once I got the moneyz and time) I need to check how acrylics behave when submerged for long durations. Of course a good varnishing is obligatory and my gut tells me it should be fine with acrylics basically being plastics...
But poisoning fish is something I do not want to do.
I'd bet it also depends on the pigments used in the paints, as things like black tend to use more natural existing pigments than the infamous yellow...

I actually dropped on of my older SM into a glas of water and intend to let him "soak" for about half a year for any visible damage, but that's not chemically proof....

Some of the tank decorations you can buy in shops are painted right? Maybe you could try to find out how they do that, since it's (supposedly) safe. Or I guess there must be some fish enthusiasts that scratch build stuff to put in their tanks. So there should already be some resource online you can find.

Also, do you put stuff in the water? Like some salts or chlorine? And do you know what the pH is? I don't know anything about soaking acrylics, but these parameters might have an influence.

It's not a marine tank, a simple fresh water tank. With that, Ph is strongly in the neutral area, around 7,1. Otherwise I'd think the fish would have other problems
Also, neither salts nor chlorine exist in any real measures for the same reasons.
In Germany, tap water is the most regulated "food" and always and everywhere save to drink when it enteres your house. If you insist of having leaden pipes... well...

Decorations are mostly made from coloured plastics. So you'd need to brake apart the plastics for the pigments to be released into the water... and plastics tend to endure a very, very long time.
I''ve done a bit of research and the consent is that acrylics are pretty safe, but you never know what some smart people in here know especially about Citadel paints. And since the life of animals is at stake if I am not thorough...

Acrylics create a hydrophobic skin when dried so you can leave it in there for as long as you wanted, the fish would be the ones that would damage them by nibbling it.

I'm not sure if the same could be said of a marine tank but a solid varnish might give a double layer of protection

Krylon Fusion clear coat.

Sounds like a cool project.

I've got the lonely mountain (from the hobbit) in my small tank. With a large Ancistrus living in the cave just inside the front door :-)

/ Regards, Fredrik

I hope you named him Smaug!

I'd imagine that salt water could have a corrosive effect, but since the inhabitans would be a betta splendens and a few shrimp I doubt much force would be applied, especially compared to knocking it over and falling of the gaming table.
Hydrophobic, xenophobic space marines. Match made in heaven.

You'll need to use the correct varnish.

I did this myself a good few years back and found that over time the paint began to bubble and peel. You'll also find that chemicals do leech from the paint into the water - dependant on number of models and volume of tank, this could be a problem.

It'll come down to investigating the right sealants or varnishes to use.

I imagine it'd be fine if you wait a good while for the paint to cure. Remember that acrylic paints feel dry within a few minutes, but it can takes days to weeks to get a full thorough cure.

That's why when people are trying to get a super good polish on paint they'll often wait a month before doing the polish because the paint gradually hardens up and hard paint polishes better than soft paint.

If fully cured paint still doesn't work, maybe just wait for the paint to fully cure and hit it with a varnish that is water resistant, say an enamel or an epoxy enamel.

In the UK you can get a special waterproof varnish for model boats, it would certainly protect the paintwork and I would guess that it's not harmful to fish if it's intended for use in lakes. But that's just a guess, I'm certainly not an expert!