I have a custom build tank sumped, which capacity wise comes out at aroumd 293L, I'm guessing with substrate and rock that'll be closer to 270L? I can't recall weight wise how much rock I put in so that has to be an estimate. Due to various reasons I've been struggling to schedule and do the changes so have been doing 60L every 2-3 weeks which hasn't been ideal. I'm torn between the AWC28 model and the AWC53. I don't want to be costing myself 51-53 litres of salted water a week so would use the larger unit every 2 weeks. The smaller one I'd use weekly but I'm not sure I'd be hitting the golden weekly 10% rule with that? Anyone with experience of the AWC units in a similar sized setup? Recall there being a fair number of 270-300L setups kicking around?