It won't kill your fish if they tolerate high pH. Texas Holey Rock is limestone used in lots of African cichlids aquariums. It's all about the pH.

I agree. People actually add limestone to ponds to raise the ph. Koi and goldfish prefer high ph between 7.0 and 8.6. The maximum level that limestone can alter water is 8.3 I can think of only two reasons that limestone might create a problem.
1. It was already contaminated with something else.
2. The pond is overstocked which causes high ammonia levels. Ammonia is extremely toxic to fish and toxicity increase as ph goes up.

I bought 2 big gravel trucks of large limestone rocks and after that the man building the pond said they would kill my fish. I couldn't use them. So to keep my husband from choking me...I put them around each of my flower beds and the backside of the waterfall away from the water.

http://www.aquariumadvice.com/...

I've been doing aquariums since '98 and have worked in fish stores. More than likely, the rocks were treated with something to keep them from decaying because limestone dissolves in water after a time. But killing your fish? Not going to do it.

I had heard that sea shells in a fresh water tank were similarly dangerous. Not so?

PPal - Not so. Sea shells just change the pH. Too much calcium raises the pH. Most aquarium fish available today can tolerate a very VERY wide range of pH (from 6.2 to 8.2), but in order for them to be "optimal" the fish should be kept in the pH that they're from in the wild. For instance, neon tetras are from the Amazon river. The pH there is very low, between 5.5 & 6.5. However, they live quite happily in "neutral" pH of around 7 to 7.5. Higher than 7.5 & they don't thrive as well. African cichlids from the 3 Great Lakes of Africa are all very high pH needing fish. For my Africans, I use either puka shell (crushed oyster shell) or crushed coral for the base. Their pH is at least 8.0, but if I wanted to breed them (which I do NOT 'cuz they breed faster than bunnies), I'd raise it to 8.5. All depends on the fish.
A single scallop shell decoration in a 10gal won't affect the water a heck of a lot.. a negligible amount. Now if you put in a huge conch shell in a 10 gal tank, after about 6 months it could change the water up to 2 points on the pH scale. It also depends on your native water. My water here in CO is naturally high in pH due to it coming from the mountains. Straight out of the tap it's already about 7.8. If I wanted to breed discus or angels, I'd have to either use RO water or use a softener (or there's chemicals to lower pH, but I don't like using chems). But, of course, one thing you have to look at are the shells themselves. If you went to the beach & picked them up, I'd soak 'em in a 5gal bucket w/1 cup kosher salt mixed in for about a week or so before dumping them in your tank 'cuz of the pollution from the ocean. If you're buying shells, make sure they're not lacquered. Do NOT use dried sea critters (starfish/sponges) as aquarium decor as they'll dissolve quickly & make a NASTY mess, plus possibly kill your fish & contaminate the tank. Decor in fish stores for the most part are OK to use. What I do is, if I'm not sure, is the finger-nail test. If I can scrape parts off with my fingernail, it's not gonna be fish safe (paint flaking off). And since most decor is make in China, who knows what chems they used in making the decor. Ceramic decor is best if you can find it. They're usually older, but they're the best. I personally love glass blocks as decoration. Melted Coke bottles make great decor (look at this picture in the background, you can see 3 melted glass blobs as decor in front of the "Jaws")

Thanks for the detailed info. I know what you mean about ciclids I had several given to me and they rapidly overpopulated a 275 gal. water trough!

I don't usually put any ornamental items into my aquariums - actually I mainly decorate with algae.

I edited the tip just a bit to make it a little less absolute.

Thanks Dave!!!

I have african cichlids (lake malawi type) in my hall pond and they do breed like bunnies. I had 10 to start with and this last week I had to empty the pond for some work and pulled over 50 out. Some have died from the shock but so far I'm keeping most of them alive in big plastic barrels with an air pump. I also bail about 5 gal a day from each barrel and add new water. It's exhausting and I hope I can put the pond back together again soon. My PH is very high, about 8.5, the water from the limestone aquifer here is naturally high and then the pond is concrete which adds to it. I also have large chunks of limestone in the pond as decoration and for the fish to hide. So yeah, it's the PH not the limestone.

I'd love to have Discus in the pond but I'm afraid they won't thrive in that high a PH. I have angels in my outside pond and they are fine, over 4 years old and 6" high. No babies, they lay eggs and try to protect them but the plecos in the pond scarf them up.

Skiekitty said:(look at this picture in the background, you can see 3 melted glass blobs as decor in front of the "Jaws")

I like the other three blobs in front of the dog.

I have been using medium sized coral rock boulders (limestone from rock pits in South Florida), for my waterfalls and borders around my plastic ponds for a few years now. I also put a concrete cinderblock in the pond to give the fish a place to hide from raccoons. Never had a problem with the goldfish health, they seem to be thriving. Good tips from all these posts - makes me want to have an aquarium again!

I have limestone on my water fall. Had some problems last year and lost 3 large gold fish and a large koi. Could it be the water fall?

ron

I've been told not to put seashells in an aquarium because they leach calcium and alter the pH, so ... maybe?