I want to make an easter island style head for a tank, the ones I've seen for sale aren't the right size and shape. Can I cast one in cement/fibreglass or would this be dodgy as they will give off chemicals into the water?

I'm afraid cement and fibre glass are both horrible for leaching chemicals and will turn your tank into a toxic soup.

I'm not sure what would work for this particular project. There are some cement mixes and sealants that could work... but I don't know what they are.

Thanks Eagle, I wonder what the ones in the shops are made from?

The ones in the shops are made of a special fish safe resin. Not sure where you can get it from. The only problem with making your own decor is that you have to make sure that every step of the process is safe for the fish. It may be OK that the resin is fish safe but the moulds you make them in must also must be fish safe as well as the lubricant used to help get the end product out the mould. If not these items can contaminate the resin and that in turn ruins all your hard work. The majority of moulds are made of latex and if not sealed properly can have disastrous effects on fish. Some humans are allergic to latex and can actually die from shock to the system from it. So imagine what it could do to fish. So before making your own decor you must make sure that all equipment and materials/chemicals are safe for fish not just the end product. You could always find a company that makes aquarium decor and ask them what they use or if they could make one for you. But that could be quite expensive.

Just for the record, most cements have a PH of up around 13 so this would turn your water into liquid drain cleaner. As EagleC has said that is a big NO. Fibreglass contains other chemicals that are toxic if subjected to certain conditions in the water above and below neutral of 7.0.

Thanks Jaguar................it's becoming a minefield innit?
I think I might have more luck with ceramic (pottery) and then getting the sucker glazed, what do you think?

Have added this to the 'how to make your own ...' thread so now the gauntlet is thrown down - get that potter's wheel dusted down and 'unchained melody' on the radio!

Ahh, Suey.........you're obviosly overflowing with romance, watching me perform on the wheel with incompetance, frustration and some theraputic swearing will soon evaporate those notions.

But I did like 'Meet Joe Black'

Was there pottery in Meet Joe Black? I don't recall any - mind you, the only time I attempted to watch it I was rudely interrupted by having to set up a 'sin bin' tank for my late veiltail who WOULD NOT leave my old singletail alone and thus had to spend the night in solitary so we could all get some sleep So my memory of the plot is missing large gaps!

I would have thought terracotta style stuff should be ok, you can use terracotta plant pots in tanks after all

Potters clay and china clay has some nasty additives in it to stop it from going off to quick and other anti-fungal stuff also. The glaze can crack under the constant temperature changes and the clay will break down causing PH problems of all sorts. Clay has a high PH that sits in the alkaline regions of about 9.0 which is where most fish will die. Also a lot of clays have an undesired habit of soaking up heavy metals and other toxins then leaching them back into the water, such as lead for example. So maybe potters clay and china are not a good idea. They do use clay for Koi but this is added in a soluble form and is totally different to modelling or potters clay etc.

Oops - scrap that then, maybe my romantic pottery notions are running away with me! But you can put terracotta pots in tanks can't you, so I'd have thought terracotta was ok Better safe than sorry though.

What sort of size were you looking for?