Backyard Aquaponics
A place to discuss aquaponics

Metals and AP water are generally a no-no
The primary culprit here would be zinc because galvanized tanks are just so easy. I know many people have seen pictures of AP systems that appear to be built of galvanized tanks. In some cases, they are actually plastic tanks that just looked like corrugated metal, in other cases they are zinc coated steel tanks that are also aquaplated (lined) on the inside with a fish safe plastic coating. Now I know some people have managed to keep some fish alive in systems that are built in galvanized tanks but as a general rule, Galvanized tanks are not a good choice for Aquaponics.
Check the coatings on metal/enamel bathtubs because it is hard to know what types of metal could leach from an old tub with chipped enamel.
Copper is also bad for many types of fish, I don't recommend copper or brass piping/fittings for an AP system.
Aluminum is also a bit questionable.
Steel or Iron would of course rust away quickly, while it might be useful to put Iron into your growbeds, you do want to make sure it won't also leach toxic amounts of other metals into the system. I would not choose to use Iron plumbing for an AP system because I wouldn't not want my plumbing to rust out.

Limestone!!!
Not all gravel media is made the same.

Limestone and Marble will keep pH really high and this is not such a good thing. The cheapest pea gravel may not be the best choice.

When you go to buy gravel, please ask questions and if you can't get the answers, see if you can get a jar full of the gravel in questions and take it home for some pH testing.

More surface area and rough materials might be good for bacteria but smooth river gravel of the appropriate size will make perfectly good aquaponic system media.

You want pH neutral media.

Easy test to see if a rock is limestone is to put it in a jar of vinegar. If you get bubbles, it's limestone reacting with the acid to make carbon dioxide.

Just a warning if thinking aquaponics say good by to all you spare time for the next 2 years after you build the first one you are thinking of the second and third
You will dream about ap
All your friends you will bore sh;:less about ap
You will get up at 2 am just to see if everything is ok
The wife will get grumpy cos nothing is done about the house
When you go on holidays you will ring the neighbours just to check the pumps are still running
But after 2 years the obsession will slowly wear of and the system will look after its self [ like it has the last 18 months ]
And all you have to do is feed the fish if you think of it and pick the veggies

I think that basically sums it up perfectly Milne...

The systems do really start to look after themselves after a few years.

My advice (in addition to lining bathtubs) is to keep pipes as short as you can. Put your growbeds as close as you can to your tanks.

The problem I think I've had with my system is that I've got a good 6m of pipe (with bends etc) between my growbed and tank, and there's a lot of pipe for something to go and get nasty in.

Up toe here:
You will dream about ap
All your friends you will bore sh;:less about ap
You will get up at 2 am just to see if everything is ok
The wife will get grumpy cos nothing is done about the house

I have yet to experience the rest...

Always upsize your piping. Gravity drains (like from a CHIFT PIST fish tank to the grow beds needs to be really big in relation to the pumped pipe feeding into the fish tank or overflows will ensue.)

I can overflow my big fish tank if I feed 100% of the water from my pump into it and the pipe feeding into the tank is 1 1/2" the drain line is 3".

If you up size the plumbing to and from a pump, you can often increase efficiency. A little pump that has only 1/2" fittings can run better when hooked to larger plumbing and the build up of bio-slime will be less of a problem in the bigger pipes.

Never restrict a pump, it is hard on the pump. If there is not enough flow getting into a pump that is bad and can burn out the pump quickly. And don't go putting a ball valve on the end of the only outlet from a pump as there is no good reason for doing that. If you must reduce the flow into a fish tank or grow bed, make sure you have a bypass to send the extra flow somewhere rather than just restricting it. It will add more air into the water and keep the pump happier.