Tank Mates For Bristlenose - Littlefish

Seriouslyfish describes BNs as being 'relatively peaceful but territorial with conspecifics and similarly-shaped species.' So should be fine with any fish that won't bother it.

However, for Penguin tetras it says 'always buy a group of at least 6 of these, preferably 10 or more. It’s a schooling species by nature, and will fare much better when in the company of its own kind. Any potential for fin-nipping is also limited when it is kept in sufficient numbers as the fish tend to concentrate on squabbling amongst themselves. Like most tetras it actually looks far more effective when maintained like this anyway.'

This advice applies to many schooling fish, including the notorious Tiger barbs, which do not tend to bother other tank mates when kept in large enough numbers.

Don't know how many Penguins you have, but upping their numbers may solve the problem.

I have 10 penguin tetras in the tank with 10 glass bloodfin tetras and 5 hatchets.
The general mood in that tank is active, with quite a lot of chasing and showing off amongst all tetras, but no actual damage. I don't know if things would be different now that the plecs have grown quite a bit, and I don't know if it would be worth trying to re-introduce them.

I don't have any experience with cichlids. I've had a quick look at the profile for the firemouth cichlid and they are attractive fish. Think the tank may be a bit on the small side though (might have to consider them for another tank  )

Most things go with BN's so not exactly limiting yourself.   For something a bit out there how about schools of threadfin rainbowfish and green neon rasbora?

For me it'd be guppies as I have soft water and I think they are lovely fish.  With your MTS I'm going to assume you have them already or you don't want them. 

I personally like the Indian Glass Fish but couldn't find a potential supplier anywhere before I found out my water was soft.  Possibly because of the current trend of dying them different neon colours.  I'd want the natural variety.

You're in good rainbowfish territory with the hard water though.  Most of the larger species can be counted out.  Maybe something like Celebes Rainbowfish and/or the Madagascar Rainbowfish. 

One day I'll do a Madagascar biotope they have some interesting panchax and gobies too that you may want to look into.

I do like rainbowfish so of all choices above I'd go for Madagascar Rainbowfish after guppies.

Black neons are lovely, and I think a large shoal of them would be beautiful. Lemon tetras are also meant to be fantastic in large shoals. However, your water is indeed quite hard. X-ray tetras ought to be fine with your water hardness but I find mine very boisterous - whether the males are sparring or whether they're chasing the females, and they inhabit the lower third of the tank and often used to knock over the poor pygmy cories in the process, having no respect for those in their way, so I'd probably caution against them and advise on a species that occupies higher levels of the tank. I'll have a further think of suggestions...

I've been having a good old think about your dilemmas.

Would it be worth seeing how Harry The BN Plec gets on again in the South American tank? If the situation works out alright, then you could use the 80cm, 125-litre tank for something that suits your water hardness. If this doesn't work out or you are concerned about him by re-introducing him there, then not worth going down this route.

If the above situation doesn't work out alright, or you are too concerned about his potential welfare to take that risk, then that potential query will no longer be hanging over your head, so time to work out what tankmates to accompany him in the 80cm/125-litre tank.

In you want to keep it South American themed, then I think your black neon and lemon tetras option would look stunning as his tankmates. Alternatively, you could go more in line with your water parameters but that would mean accepting that it's unlikely to be a South American themed tank. Out of interest, would there be room in the tank of penguin tetras and hatchets for the lemon and black neon tetras, or do you think that might overcrowd it? Just having a think, in that case, about a potential Asian tank for the 125L, with the Indian glassfish in there and potentially something like cherry barbs.

What started out as an attempt to put this into an algorithm-type format to make the decision easier for you has probably ended up confusing you further... 

PS. One of my favourite fish http://www.seriouslyfish.com/species/sahyadria-denisonii/ - unfortunately too large for your 125-litre - would be perfect for your water, so perhaps a future fish for you to consider if you liked them...

Ok, so there is enough room in the tank for both Harry and the BEA.
The question now is will it be safe for them now that they are bigger, or will they by nipped by the penguins?
There is a pile of spiderwood in the tank, with space underneath (where the shrimp hang out), and I could take out the rocks and replace them with a bogwood pile for Harry & BAE, but is it worth trying to reintroduce them?

The alternative is the 125L tank, probably go with the black neons & lemon tetras, and soften the water a bit with the addition of some bottled water or RO.

Any suggestions and/or comments would be appreciated - even if the comment is "don't be a numpty Donna, if the penguins nipped before then they will nip again, however much wood you put in for Harry & BEA to hide under". At least that would make the decision for me.

@Littlefish Angles, Buenos Airies Tetras, Clown Loaches, Corys, Cardinals and BLack neons go well. My 2 are active and i see them very often in the 190L

I would forget loaches for a tank that size, except for the rosy loach which might be too small to cope with the bn and bea. It is OK in water up to 15 deg hardness/268 ppm. You might just get away with dwarf chain loaches as the tank is the minimum size for them - but they are very expensive. All other loaches get too big.

On a tank size basis, most cories would be OK but they tend to be softer water fish. Some species are OK in harder water, you'd need to research which ones before buying.

I was considering dwarf chain loaches for my 200L/100cm tank with perhaps some pyjama catfish/pictus catfish/spotted hoplo, basically a set up where the bottom dwellers are monochrome and the emphasis is more on the markings than the colour.
I would consider cories with the bn & bea, because they are lovely. I hadn't thought of it before, just need to find something suitable, and hope the catfish aren't too bothered by them - the peppered cories in the temperate tank are quite active, and even more so now that there are more of them in a larger tank.
As for the lemon tetras, I've liked them for a long time, even in the shop. I've seen pictures of them settled into a tank and I agree that they are stunning.
As daft as it sounds, I've got a bit of a thing for colour coordinating fish to some extent, which may stem from the South American tank where all fish are silver with a tiny amount of either black (hatchets & penguins) or red (bloodfins). So with the current tank I thought black neons would go with how dark Harry (bn) is, and the lemon tetras would tone with Bertie (bea).
Before anyone mentions it, yes I know it's a crazy way to choose fish (and I don't know why I do it).