Yes, a 45g is to small for one Fahaka, although it would do for the first couple of months or so.
Fahakas are VERY aggresive, are you prepared to devote a large tank to just one fish? We currently have a four inch fahaka in a four foot tank and nothing else. She will be moved to a bigger tank later.
Yes, sand is a must. That's Martha, our fahaka in my avatar, buried in sand.
The Mbu is a no-no, unless you have room for a 12 foot by five foot tank.
If I were you, I would go for a couple of South American Puffers, they are much smaller and can be kept with amoured plecs and other large, tough fish.
The only problem with SA Puffers is that their teeth grow very fast and may need regular trimming... not a task for the faint-hearted!
Other options, if you can find them, are Dragon (humpback) puffers -t.palembangensis which get quite large, but because they are lazy could be kept in a 45gallon, or Target puffers t. cochchinchinensis which stay fairly small.
All the puffers that I have mentioned are TRUE freshwater fish and do not require salt in their water. (although a little is sometimes beneficial)
All the puffers mentioned will require specialist foods such as mussels, cockles (in shell when the fish are bigger) whelks, prawns, lancefish etc, also ideally, live crayfish, lobsters & large snails. Some people also feed live fish.
Puffers will rarely take flakes, pellets or tablets.
) fish out there. 
), I guess he'll get too big for my tank then, seeing as how he'd be half the length of the tank. 
What about the Ascellus' (sp?)
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