well, the main reason is the fact that glass is not that flexible... and as I wasn't sure of my basement floor perfection level, I had to compensate it by a "fluid" concrete, which will regulate his level by gravity... when poured into the wooden box, the concrete was also "vibrated" to condense and to fill every imperfection of the ground... I'm not sure to be precise in my wording, it becomes a bit too technical for my English skills...
on the topic filtration, I mean poor in both ways : turn over (1 per hour) and filter media capacity (90 liters of fluid sand only)... But I have no problem of nitrates or whatever else, as I continuously change water (drop-by-drop) to 4m3 per week.
Finally, last time I checked the size of Arowanas, I took out of water the smallest (which was O. ferreirai, male) for selling it. It was back in May 2014 and it was 79 cm long. So I can estimate that my big females are now more or less 90-95 cm long. which is more or less 37.4 US inches... They are exactly 3 years old this November 2014 (as I bought them with yolk sack still on them, I can be quite precise).