Of that list I've only raised the black arowana to "maximum" captivity sizes but I have raised or researched most of the others:
1) Wanted to keep one but never had the opportunity to buy. From what I've read really more of a a single-specimen tank kind of fish due to it's aggression. However, with a 180g (6'x2'x2') though I think it may be possible to "over-stock" on similar sized fish to keep aggression at a minimum.
2) Lots of different species in this category, you'll need to be more specific.
3) A community fish but IME typically better as a angel only tank although you can mix a few other species but YMMV. Tried keeping them before but they were a hassle as they stressed more easily than my other fish.
4) Possible to raise in a 180g (6'x2'x2') for ~1.5-2.5yrs depending on water conditions & feed rate but you will have to upgrade ideally to a small pool sized aquarium. You could get away with a 180g-240g for life but you would need to custom build for a wider footprint. Typically a single-species tank due to it's growth rate and aggression. However, it's not uncommon to get a black arowana that is tolerant of fishes of a large size relative to it's own length. These tolerant specimen will still bully their tank mates though.
5) Never researched this species heavily or had one.
6) Like the black arowana but more aggressive against tank mates; must have a pool-sized aquarium built and ready if you plan on keeping it for life. If you're sticking with only standard sizes you could get away with a 600g long-term but not for life.
7) Another single-specimen type of fish although again you could try to "over-stock". I've heard of 20"+ specimen in the wild but I've never seen one even close to 20" in captivity.
8) Was skipped in your post.
9) Possible to keep a small adult community (3-5) of these in a 180g (6'x2'x2') for life although I would suggest bare bottom tanks in that case or very minimal decor. Also they get really chunky and IME have a habit attempting to eat any fish they think are smaller than themselves. Heard about them being very aggressive for some keepers but never had an issue when I've tried raising them.
10) Never was able to keep large plecos, my fish kept killing them so I can't really help you there.
As for what species on this list could co-habitat IME & IMO:
- Your best best is keeping the Red Wolf-Fish with some pike cichlids of similar or slightly smaller size in an overstocking scenario.
- Most fish aggression is stems from either territory defense or feeding behavior. In theory if you add enough fish in your aquarium, regardless of their temperment, they will be too busy trying to look for a territory to defend resulting in near 0 deadly aggression between all tankmates. The downside to overstocking is of course the extra bioload in your system resulting in more WCs being required.
- Aggression will still be present in these types of systems so most people will also include some sort of dither fish/species
- Angels can't really exist with any other fish on this list although you could get lucky with the bichir or pleco.
- Unfortunately angels are typically sensative to water conditions so you'd have to have a pretty high WC
- IME angels also don't do well when they aren't schooled.
- African Pike could potentially be kept with some larger pike cichlids but you would have to overstock to keep aggression down to a minimum but even then that doesn't guarantee no deaths. Since this species can grow 18"+ it really is more of a single-specimen type of fish unless you upgrade your tank IMO.
- Arowana and Gar can both co-habitat the same tank and have similar growth rates but IME & IMO if you're not going to keep these animals for life it's better to avoid getting these/any truly monster species at all. It's a real pain in the ass to move such powerful animals especially when they get older and you have to rehouse them either to a new owner or just to a new tank.
- Never kept any of the gar species
- I've kept all arowana species to 24"+ with the exception of Asian Arowanas
- Growth rate for the first year with good quality food + heavy WC schedule is roughly 1-2" a month upto ~15-20"+ in the first year depending on species.
- Assuming very aggressive feeding and WC schedule Silvers and Blacks can reach up to 20"+ in the first year
- More realistic growth rate is roughly 1-1.5"/month up to their first year then slightly less than 1"/month until they are around 2ft TL and then they start growing more girth like Jardini.
- IME both silvers and blacks start to feel constrained in a 6x2x2 footprint around the 18"-20" mark resulting in them potentially jumping. However this is also heavily dependant on your particular fish as most of my arowanas still grew above 2' in a 180g before I got larger tanks.
- Assuming very aggressive feeding and WC schedule Jardinis and Leichardti can reach up to ~15-18" in a year
- More realistic growth rate is roughly 1-1.5"/month up to their first year then slightly less than 1"/month until they are around 15-18" TL
- Both start getting thicker instead of longer around the 12-15" TL
- IME my Jardinis always looked thicker than my Leichardti but both species are nearly identical at a distance.
- Assuming very aggressive feeding and WC schedule African Arowanas can reach up to 15" in a year (I've heard of longer but IME I only got ~15" max)
- Fairly difficult fish to feed compared to other arowana; they aren't really surface predators. Got mine to eventually accept pellets but live food like blood worms were required when they were >10". I could have pellet trained them earlier but I didn't feel comfortable swapping them to pellets at that time.
- I suggest avoid trying to go for a 2"/month growth rate for any fish as this may result in long-term health complications to your fish; specifically fatty liver issues.
- Ornate Bichir can exist in the same tank as the arowana and gar but there is a risk of causing injuries in your idealized tank setup as their movements could cause the arowana and gar to accidently get triggered into an attack -> hitting decor.
- IME everytime I had a pleco with my larger cichlids or arowanas they would kill it but I have heard/seen it working successfully with others.
Filters
This is going to depend on stocking levels, size of fish, and feeding schedule. Most likely 1 FX6 will be enough from start to finish but you could use 2 if you really want to. Some key things to note:
- Mechanical only filteration doesn't exist; all filter media will act as bio-media
- Mechanical filters typically don't contribute much in terms of water quality due to their small pore sizes but they will increase water clarity (to a degree) by filtering larger solids out of the water.
- You can't really go wrong with any type of bio-media although there are some that are mathematically better due to providing more surface area
- Unless you aggressively overstock and/or overfeed
- Water Changes & Total Water Volume (TWV) are more important than the # of filters
- You can extend the time between WC a bit with a heavily planted tank but most monster fish do not do well in planted setups.
- TWV is more important than the # of filters because it acts like a buffer before nitrates build up to a harmful level
- research states ~40PPM before chronic illnesses can start to manifest but this field of study is hardly researched in-depth; especially for home aquariums
- arguably anything above 0ppm can have a chronic effect on our fishes; look up the posts by
duanes
EDIT: Forgot to say, consider making your own sump setup instead of using an FX6 or Oase 850. It will be a bit more work but definitely cheaper and molded to fit your needs.
HTH
