ok, so in a power outage situation, around 15 gallons will drain from the main tank.
This mean, out of the 90 gallons of sump volume you have, you can use around 75gallons of it.
So you can run that sump with 75 gallons in it at all times if needed.
So if you went with a fluidized bed, you could have a chamber for the media that holds around 60 gallons of water. Of that 60 gallons of volume, you can run 136 liters of fluidized media. Preferably K1.
If you went with a wet/dry, the only real restrictions is how much room you have under your sump. The tower can extend out of the sump if needed. But lets assume that it needs to be level with the sump. A wet/dry usually runs with about 25% water in the sump. So you could hold about 60 gallons of bio-balls in the sump.
That that is the amount of media your sump would hold in each situation.
No, to find out if this will be enough to support your stock.
K1 can easily support a waste around 5grams per liter of K1 in a day. So your 136 liters of K1 could support 680 grams of waster in 24hrs. I find that it can do double that at least, but lets go by what its rated for.
To find out how much waste will be produced in your tank, you will first need to know how much your feeding.
Typically, rays/arowana are fed a lot. Usually around 3-5% of their body weight.
A fish, on average, weighs around 10grams per sq/in of fish.
So lets stay your rays were 12" across. Averaging out their disc to be around 12x12x1.5 . Rough estimate, but we need to go by something. So that ray would weigh in around 2100 grams. Probably less, as their are no real bones in a ray. so i would say around 4 pounds(1800grams) for a 12" healthy ray. This is probably a bit off, but we need something to go by.
You have 4 rays, so that makes 16 pounds of fish.
Lets also say your arowana are 16"... so about 16x5x1.5.. so it would weigh around 3 pounds.
You have 2 arowana, so that makes 6 pounds.
you jumbo loaches, if around 8" each, weigh in at around a pound each.
you have 5, so 5 pounds total.
In total you'll have around 27 pounds of fish in that tank. 27pounds = about 12,000 grams.
If you feed 3% of their body weight a day, you'll be feeding around 360-600 grams of food a day. Probably much more.
So your k1 would be able to support this load with easy.
If your doing a wet/dry, and hold about 60 gallons of bioballs, you would also be fine. bioballs are 40-60% less efficient than K1. But since you are hold more volume on bioballs than K1, they are both basically equal in your case, in terms of what they can handle each day.
So either way is fine.
Both of these styles are not complicated. A wet dry or trickle filter, has a tower for the bio balls, with a drip plate.
A fluidized bed, gets 2 baffles.
Pretty simple lay outs for both.