We have had an aquarium maintenance business for more than a decade, and have never had this problem:

One of the new tanks we've set up is a nano cube (28 gal.) and is going VERY cloudy within 1 week of setup, for no apparent reason.

We've talked to shop owners, other aquarists, changed water (full & partial), done additives, changed salt, added & removed all kinds of rock & plants... the seahorses in the tank seem fine, but we've had no luck in getting the water to stay clear.

Several other tanks use the same water, but are mostly 50 gal. & above. We have used ceramic bioballs, plastic bioballs, have used just filter.. nothing is working, and we've been at it for more than 6 weeks.

Is this a common nano issue? We take care of two other nano (less than 30 gal.) & they don't have this issue - & use the same water! WE HAVE EVEN CHANGED THE TANK, from a NanoCube to a AquaEuro (same gal.)... 1 day, it's already cloudy!... but the seahorses still seem fine, as do the other animals & plants...

Anyone who can help. please, to give insight, suggestions... we are at our wit's end!

Tank you!

Was it clear when you first set it up and then got cloudy after? Do you test what you dose for?

they had this problem at my lfs in their display for a month or so... very peculiar. everyone seemed fine in their tank as well, just very cloudy and explanation. they were going nuts. then it cleared up as randomly as it clouded up.
my theory: its biological! i think its some inverts spawning in the water (stomatella snails, etc?) that is causing the cloudiness.
especially if you guys have done full water changes and its still cloudy...

my theory comes from experience: my 5.5g was running for its first month or 2 very VERY VERY cloudy. then, the cloudiness mysteriously cleared up and shortly after, dozens and dozens of baby stomatella snails popped up all over the tank...
just my $0.02

What is the color of the cloudiness?

Well, trying to pick out correlating factors here; you say that you've changed tanks, and the problem persists. So what are you using in the new tank that you used in the old one?

How long after setup before the animals were added?

Are you using the same plants/rock/livestock that you used in the previous tank?

What kind of plants? If you're using some macroalgae, has any of it gone sexual?

Agreed also; probably need water parameters and color...

Hmm, you've had a cloudy tank, then you changed something, then you changed something else, then you changed tanks, then you...

Dude, QUIT FIDDLING! Things take time to settle down after making changes. If you make changes, give it a month before you write off that change as useless. Even moving rocks around can unsettle a tank, just leave it be and give it a change to reach an equilibrium. Start testing for ammonia and nitrite (though it's likely that they may both be 0, indicating nothing is wrong). Do you dose anything? Don't dose what you don't test, you may be overdosing something. Unless you're dosing VSV or something (which you shouldn't do if you don't have a skimmer). Do you have a skimmer? Maybe it's something to consider.

Otherwise, just do weekly water changes and everything will level itself out in time. You have to be patient in this hobby, or you'll go insane. Or kill everything