Hexagon tank for fire bellies

Here is the thing, elaborate set-ups with deep substrates and difficult access are not practical at all and can potentially lead to many problems.
It's also important to note that these newts behave 100% aquatic in captivity if conditions are adequate which means that there will be no climbing and no real use for any terrestrial or aereal surfaces. Instead, all that volume should be filled with water if you want them to make use of it. A larger volume is also highly benefitial not just for extra space but also because it will be more thermically and chemically stable, making maintenance much easier and providing a better environment.
The stones can be a problem, trapping large amounts of debris and uneaten foods where the organic matter can rot and mess with your parametres and even, potentially, create lethal conditions.
I know the visual impact of elaborate, naturalistic set-ups is like a drug but unfortunately it requires a lot of maintenance which in such a set-up would be difficult and it's far more impractical as well as risky.
I would strongly recomment rethinking the set-up, perhaps going for something simpler with a thin layer of sand as substrate and some decoration with an abundance of plants, or if you want something more elaborate it might be a good idea to consider creating a fake bottom with glued, secure rocks, sealing the spaces in between with adequate products and preventing the accumulation of debris and potential accidents. You can do some pretty attractive designs with some easy techniques. There are examples of such things in these forums and others.

Anyway, as far as plants go, there are many that work well in caudate tanks but some of the most popular are java moss, java fern (Microsorium), Elodea, Anubias, Hydrocotyle, Vallisneria...
All of those do well in low light and tolerate the cold conditions required by caudates, plus some of those make excellent laying substrates for their eggs.

Well, personally, I can't stand hexagon tanks, they're a lot harder to use a siphon in. The rocks would be an issue with everything that requires food and creates waste, so basically everything. So I can't really point you in the direction of anything, maybe someone else could.
I have however kept bettas and guppies in hexagon tanks, but that was when I was dumb and dumped all the tank water in the toilet each month to "clean".
Personally I would sell the hexagon tank and purchase or make a 20 Long aquarium.

You are going to have the same issues no matter the occupant. The rocks are mobile so they can cause injury and they will trap debris no matter how clean you are. I sympathize, it's hard to let go of a set-up that you like, but think of it this way, what matters more, the well-being of the inhabitants or using all the rocks you collected?
You could keep the superficial appearance without the depth and crevices if you created a fake bottom and glued the rocks on top, but it would take some work.

A 20 or 30 regular tank would be better. Also u need a lid and completely secure right. Newts will climb out

jimmies1974 said:

A 20 or 30 regular tank would be better. Also u need a lid and completely secure right. Newts will climb out

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Just somethings on your enclosure, switching to sand or bare-bottom would be much better(reasons above), and is that a heater and fish? What temp is the water, whats in the tank, etc.

Heater is shut off. In less it gets freezing point

The rocks work fine. I put a glass jar for the worms. The newts go right in to eat. Works out well

Stones are usually considered a problem because of waste. Food can be a problem but not all of it. Unless your newts are somehow trained to poop in a jar, waste is getting trapped under the rocks, Personally bare-bottom or sand look much better and its much easier to clean, my siphon doesn't even suck up much sand when its put directly on the sand, then again I don't have a fancy electronic siphon. I also recommend more hides, I often see N. kaiseri enclosures with several hides and crevices and stuff.