Keeping dead coral in a freshwater tank can be given a couple of spins. As with most everything in life, there can be good news and/or bad news. :)

Some people don't want to encourage unnecessary harvesting of coral. On the other hand, if it washed ashore or has been in the family for years, use it.

There is also a "desert coral" available in garden centers around here. I worry about where this is being mined out west, but it doesn't seem to threaten living coral reefs.

I notice that it can cut fish (guppy) fins. I wouldn't keep Corys around sharp coral and I would never keep Corys over coral gravel. Corys without whiskers either lost them because of the bacteria in dirty water or they were abraded by edged gravel.

I would never, never want to use it with tropical fish from soft water habitats, often rain forest. The dissolving of the coral could create real problems.

On the other hand, coral gravel is sometimes used by rainbowfish people. They find their fry do better in water with a slight alkaline tendency. There are even killies - surprisingly, young Epiplatys lamottei and not so surprisingly North American Fundulus and Mediterranean Aphanius - which do well in such water.

I have used tufa rock to buffer Rift Lake tanks. That stuff is basically the artistic build up of calcium and other minerals on the shores of alkaline lakes in places like the SW US. Probably Oz has similar places.

When starting out, I had a pair of the rainforest Aphyosemion striatum in a tank with the Tufa rock. Next to it I had a possible pair of the Rift Lake Julidochromis ornatus. Neither tank yielded any spawns.

After reading about their habitats, the DOH! factor set in. The tufa rock was placed in the Julie tank and a small tap water change was made. The striatum had a massive water change and then the gradual addition of 20% rainwater a couple of times.

Within two weeks there were fry in both tanks. The other care, including the feeding of some live food, hadn't changed. In that case, a look at their native waters made all the difference in the world.

You are pretty faithful with water changes and probably the coral would add a few trace elements and not hurt the guppies. But please  consider my little blurb on mystery deaths associated with the accumulation of minerals in a guppy tank.

Winter Mystery Deaths (Ask Guppylog, Care Tips)
posted on 02/07/2004 15:30:19 PST

At a certain point there is an important chemical draw back to having a higher pH in a tank. Chemically inclined Listizens, please correct any gaffes I make in explaining.

Freshwater fish actually release a lot of their urine through their gills. (That is another reason why funky water burns their gills and inhibits their breathing.) What is mostly released is ammonia. At lower pH levels it turns into the less toxic ammonium, which like Nitrates and nitrites is still dangerous if a lot of it is present (not removed via water changes). Above 7.0 it remains the much more deadly ammonia. As pHs rise above 7.8 ammonia becomes even more dangerous to the fish. (I really don't "know" why, but am accepting the testimony of several others on Killietalk.)

That is why coral in a tank can be a threat if we get casual about water changes. Plants also function more effectively near 7.0 than way above or below that. Perhaps the phrase, often found in aquarium books and articles on fish care, "so long as extremes in pH are avoided" does indeed have a pretty widespread application. :)