The Optimats are useless, even on a smaller tank they cannot keep a stable CO2 level. And the DIY ladders on a tank that size are likely not ideal either.
If money is no object then the Boyu diffusers via eBay for £5 each (yes, £5!) are cracking value for an external inline diffuser. You'd work them with DIY CO2, but for a tank that size a 2KG fire extinguisher and eBay regulator would be the best bet - set you back around £75 but you'll get stable CO2 that you can alter the dosage of to ensure (with a drop checker and 4dkh water) you get 30ppm.