Insurance investigators are incredibly savvy with regards to these types of situations. I mean, you'll have all sorts of salt water damage to the inside of the house, but miraculously the outside of the house will show nothing, no dead grass or gardens, no salt damage the siding of the house, no dirt runoff from the flood etc.
If the fraud level is large enough they won't initially tell you they are denying the claim. They will start to gather evidence of the fraud. Like the square on the floor where there used to be a tank. Evidence of a large aquarium will be all over your house, from RO units, salt buckets, testing equipment etc etc. Then they'll subpoena the electric company for your historical usage and compare it with your new usage and see that there was a major drop in usage right at the time of the loss. They will also get your credit card history and banking info and see you buying all this stuff that isn't there.
Then they'll send you a letter asking you to verify all your claims, sign it and mail it back. Once you do that you have risen to the level of mail fraud. So they take the Insurance Fraud and combine it with the Mail Fraud and take you to court with a possible 10-25 year sentence and some large fines.