In-ground composting of a galvanized steel stock tank?

About 25 years ago, I bought a shiny 2.5' deep galvanized stock tank to use as a soaking tub. A few years later when it became dull and algae moved in, I dug a 10' diameter hole for the 6' diameter tank and leveled it with the surrounding land to create a water source for goldfish and wildlife. Now, the tank is seriously corroding - springing leaks - and has come to the end of its life. Rather than buy a liner, silicone, JB Water-weld or other products, I would like to let the tank return to the earth safely in its current resting place. From fish and frog habitat forward, how does one safely and humanely encourage the decomposition process of galvanized steel in the landscape?

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The backfill around the tank is fine desert sand and silt. The water in the tank holds fairly steady 6” below the water line. This morning, I plugged the holes above the water line with cone-shaped / tapered pieces of cottonwood screwed into the holes from the inside out. The plugged holes range in size from 1/4" to 1" after flicking away the rust shards. Water pressure will hold them in place. Now the tank holds water all the way to the tank lip with very slow drips as the dry cottonwood plugs absorb the water and expand. This fix is obviously temporary since the 4 holes are rusting and many other pin-holes are developing as the zinc and steel degrade. But I do see that the plugs will enable a transition from a pond to a bog garden.

Unfortunately, after 3 hours, the pond has returned to its pre-plugged water level. I had to address the problem quickly because small animals go to the pond to drink and cannot get out of the tank. To prevent the animals from drowning, I gathered branches, submerged them in the water, wedged them under the plant muck to create escape ramps.
Looking at the large branches resting on the pond lip has led to another idea. The wood soaking up the water and muck in the subterranean tank reminds me of a hugelkultur trough. I am intrigued by the possibility of making the transition from pond to bog to compost using the hugelkultur method. Any thoughts on this approach to long term in situ composting and transforming the area, galvanized tank and all, into a slightly raised garden bed?

Maybe the liner solution is the best since you are concerned about animals not getting a drink.

Thanks for considering this problem John!
Yes, the liner is the right approach to preserve the pond and keep serving the animals that visit daily. The reason I don't want to go with the liner is that the animals - coyotes mostly - essentially need the equivalent of a few dog bowls. Birds need shallow bird baths. I've really over built!
The maintenance of a pond in full sun in the desert is more than I want right now. Yes, I am concerned about toads, lizards, and snakes but they too can do fine with shallow drinking options. So I could fill in the galvanized tank and save myself a lot of money on the liner (the pond is actually 8' plus the edge and pond depth which is $$$. I'm ready to give it up and offer water in smaller servings.
Unfortunately, despite my best efforts to catch them, I dread killing the snails, fish, and frogs that will hide when I transform the ecosystem.

What about a shallow liner with rocks etc and a shade cloth over the water?

Okay John, let's say that I do as you suggest: "a shallow liner with rocks etc and a shade cloth over the water." I envision a barely perceptible depression that can flow in any conceivable shape; one that fits with the way the landscape has evolved over 25 years.
With all the possible choices, the pond shape that I am least likely to adopt today would be that rigid circle that appealed to a more perfectionistic time. Now I see the flaw in that demanding shape which failed to gently evolve alongside the land. What I've come to love over time are those natural features that appear to have grown out of this desert place.
Which means that the perfect circle needs to go. But it is virtually impossible for me to remove that giant object. So instead of removal, I want it to melt back into the land: to bury it or compost it, or transition it into something more consistent with the emergent landscape rather than the perfect circle that I imposed upon the land decades ago. I'd like to accelerate the decomposition of the relentless circle.
It may take years but once that limiting circle dissolves into the past, the garden will call forth the next element: a small tree, a little wildflower patch or a shallow rocky water garden.

For the plastic fish pond installed by previous owners, I built a series of shelves out of scrounged, rocks, split limestone, and old bricks. This gives all creatures access to the water regardless of the fluctuating level in the pond.

It's also pretty easy to create a floating wooden raft of rough, scrap lumber that allows access to water without drowning or trapping the local fauna. This is a 15 minute job, or 5 minutes if you chop up an old pallet.

Water is life. It's worth patching up the holes.

Thanks for your encouragement to patch the disintegrating tank Douglas.
The tank is beyond patching. What appears to have happened is the 6" portion of the tank that is not buried has been pushed out by a thick layer of ice and compromised all metal above ground level. This morning I now count more than 20 leaks. The metal is rusting and brittle.
But what I am thinking of doing to allow myself more time to consider composting options is to cut off the top 6" portion of the pond. I will use the reciprocating saw with a metal cutting blade to take off the top ring down to the lowest leak ~6".
I'll cover the sharp rusty metal with rubber U channel edge protector for sheet metal.
So I'll have a round pond as the transformation of the tank continues.