This is basically a numbers game Curtis.
Firstly, a 72" x 18" x 18" is 100 gallons, not 125. A 125 is 24" deep.
You don't need extra depth anyway and the lower tank works better for you in this application. A 100 gallon tank can still be a totally awesome River-Tank.
My original concept was to recreate natural conditions for these fish as far as is feasible. Remember that we're talking about water flowing as much as 1 meter a second. That means you would have difficulty standing in it at any reasonable depth. Moving water like that in an aquarium just isn't feasible so we have to settle for a lesser flow.
The possibility of using 4 Maxijet 1200s is because you have them or that was just an idea? Total setup cost is dependant on knowing what you are trying to acheive and what your pocket can stand. The bigger we go in aquarium capacity, the more expensive it gets. This is because we're talking about total water turnover. If we realize that just about anything we can fit in a conventional aquarium is substantially less than nature provides these fish, then under sizing the pumping equipment lessens that ability to provide a facsimile of nature.
My first River-Tank was around 52.6 USG and the R/T manifold was powered by two Aquaclear 802s at a total approximate pumping rate of 1800 gph. This gives a turnover rate of approximately 16 times per hour in an empty tank. Reduction in water capacity due to decor means that the actual water capacity turns over more than that assuming the pumps work at their most efficient.
If you look at Emma's new River-Tank setup I think (please correct me Emma) that is a 48" x 15" x 18" tank which is around 56 USG. Powered by 3 Maxijet 1200s that works out at bang on the 16 times an hour minimum I would always shoot for now because I know it works.
With 4 1200s in a 100g tank you're looking at 12 times per hour which is below the minimum by a fair percentage. 4 pumps squeezed into 18" of width might not be doable either. Look at 3 in Emma's tank.
You have no room due to the length of each pipe 'T'.
So choosing pumps comes down to calculation.
100 x 16 = 1600 gph required.
I like Aquaclear 802s (Aquaclear 70), they work in this application great. But at 400 gph we would need 4 to be right on the money. No space. An alternative is 2 Aquaclear 110s (old 901). These pump 900 USG/hr so you would end up with 1800 GPH which is even better
Only problem is I look at prices and compared to the Maxijets you're talking twice the money. But if a job's worth doing it's worth doing right.
Power costs money unfortunately. The 110s are just an example. I'm not sure what other options there are out there but the things you need to look at are bang for your buck (in other words, pumping capacity balanced against cost) and actual adaptability to the PVC pipe fittings.
As regards the manifold itself I would build it exactly like Emma's new one with 4 longitudal tubes because this will aid water movement at these sort of flow levels.
It is quite probable that with multiple pumps or even just 2 big powerheads that the heat-sink effect from them will actually heat the water up over the ambient air temperature. I would expect at the temp you're quoting that the tank would probably maintain a temp in the low 70's which would be ideal.
Plants that will work are
Anubias, Java Fern, Java Moss,
Bolbitis. If it's not in the direct flow from a powerhead
Vallisneria will probably work too. Others have experience with various plants in River-Tanks and can advise.
Martin.
My moss always seems to do better in a cooler, higher flow area..
The tank is a Fluval Roma 240, which measures 120cm long x 40cm wide x 50cm high (in old money that's 47.2" 15.7" x 19.7"). As well as the 3 Maxijet 1200's, this tank is filtered by an Eheim Professionel II 2028 (rated for a 600 litre tank - this tank is 240 litres) with the spraybar return positioned at the same end as the powerheads.