Yes I am serious, though, they looked bigger in the pictures. With them (now that I know) being 3-4 inches long, that should suffice.
55 gallons is the minimum to keep nitrates under 20 ppm with largish water changes each week and good filter maintenance. I would personally say 50 gallons per oscar and so does everyone on another forum that I visit that all have thriving fish.
Why wouldn't you recommend 80-90 percent water changes? I do 150% water changes (drain as low as possible without the fish being out of water, refill to 75%, drain again as low as possible, refill to 100%) to set the nitrates back to zero every week.
As long as the nitrates stay 20 ppm and under constantly then I will not bug you...
How are you treating your water before it is changed? I am on city water and we have high amounts of chlorine in the supply.