Fish pond capper

Hi guys, I'm a newbie so apologies if this is the wrong forum..first post.

I have an ornamental fish pond in the yard, rectangular, approx 6x1x0.5 metres, Block construction, sealed internally, pump/filter/lights, clad in random sandstone. Complicated by the fact that the fish are already in it...it's been a work in progress. All looks great but needs a capper to finish it off. Have looked at various stone or cement options but nothing suits the boss..or me to be fair. As the house is a mix of natural stone and timber I was thinking of using merbau...to match the nearby deck.

So I have some questions....
Will merbau be ok close to water (minor splashing from pump/filter heads and rain)?

I was looking at 190x45 but can't get the 6m lengths in one piece. Any suggestions on joint to keep it aligned along the long sides? V groove, pinned with stainless steel. I figure dowel or biscuit is probably not be strong enough and may rot out.

How about the corners? Mitre would look good but I ain't the greatest woodworker around on bigger sections. How about a half lap?

I know merbau will bleed but stone is sealed (fish aren't though). Any suggestions to reduce bleed? If I keep it oiled will it settle? I think I read on the forum somewhere that hitting it with a deck cleaning solution would help speed up the initial bleed, but then again it's not decking boards.

If not merbau what elee is around at 6m larger sections?

Lastly, any suggestions on attaching it to top of low block wall. It will be a casual seat and will only have a small overhang over outside of wall. Don't want to get too carried away with achor bolts and make the wall leak.

Hope this isn't too long a post. All your good advice very welcome.
Thanks

I'm not sure how the fish will cope with either the bleeding of tannins out of the merbau or the sealer to stop it from doing so, some things entering the water would be like a toxic gas for us in the air

Unless you pre-treated it all before installation, that may be better then just touch up any areas that need going over.

You could also do a border in slate or spall rock which may tie in with the pond better then have the deck adjoining that. One again though you will have to watch cement/mortar etc doesnt get in the water that can kill the fish (although goldfish are tough buggers, they are related to carp)

fish are koi and gold?

Tannin is ubiquitous material that also comes out of gum leaves etc - any pond that is black in colour is because of tannins - and i've never heard of black water killing carp - i had my carp in the pond and the pond got tannin stained and they are fine.

yes clean merbau with deckclean (thats deck), napisan, blast it a few times with the hose, let dry then coat with something - you could give it a marine varnish or oil - give it a few coats before installing - just because the tannin is still a PITA and better to be avoided if you can.

personally I'd think about whatever natural stone is on the house I would cap with - friend has used 50mm sandstone slab which looks good. definately mitre the joint butt joins just dont look like they are done with luuuuuuuv!

Thanks. Pond is pretty well bio established with plants and the inevitable bit of muck and sediment so I was hoping a little bit of tannin wouldn't stress them too much. The fish are goldfish...breeding like rabbits so I guess they're happy. I was just going to oil the timber before I installed it. The random sandstone only goes up the outsides, inside is sealed with several coats of paint on membrane. I would need to be careful if I started mortaring stuff. I know what you mean about the butt joints. We have lavished too much time and money on this 'feature' already so I don't want to spoil a good job. House finish is Fremantle limestone....great for house but wouldn't work for pond capper. Hadn't thought of spall rock. Will look into that.

Do you reckon I'd get some more replies if I cross posted to another forum? Is this area too specific?

My nephew used recycled hardwood sleepers on his pond. See pic.

if sandstone up the sides, thats what i would cap with - though rather than mortar, use an extrenal good quality tile glue (maybe the stuff recomended for pools would be a good start - as that will be thin and also a good bond - just cantilever 50mm each side with 250*50mm sandstone slab - thats plenty wide enough for occasional sitting for 10 minutes or so

I can confirm the fish won't mind tannin in the water (in fact the carp family like slightly acid, dark waters). Vegetable matter etc won't harm them.

They're pretty tough as PlannedLandscpe said, and if they're breeding, they're happy.

I'd be wary of any chemical used to treat the wood - this is what will kill the fish if it gets in the pond. The pond is essentially a closed system, any contaminant that gets in will get in the fish. So any oil, varnish etc could be a problem, if it gets off the wood and into the water.

And as mentioned before, lime from cement, limestone etc will harm them too, given long enough or high enough dose.

It also depends on how big the pond is. The bigger the pond the bigger biological system it is there for it is able to tolerate more foreign toxins too it before it gets to the levels that it will effect anything.