Sorry but, I don't think 30% water changes once a week will cut it. 30% 3 times a week or 50% twice a week will keep the guys in the pink. Look at it this way; You have a 100gal tank where the bioload produces nitrites in the range of 10ppm per week. Once a week, 30% of the water is replaced. That removed 3ppm and leaves 7ppm at the beginning of the next week's waste development. At the end of the week, with no further water changes, the tank will contain 17ppm of nitrites.
Granted, 10ppm nitrite development is quite high. But, with large cats, it's within the realm of possibility to have levels like this (especially if just one cat regurgitates a partly digested meal).
Unfortunately, physiologically toxic levels of nitrite goes unseen right up to the point of no return (brown-blood disease) on large cat species. You'll think your water changes are sufficient mainly because "I've done them the same way for years and they've been fine". But, the fact is the maintenance has to increase parallel to the rate of growth of these cats.
Granted, 10ppm nitrite development is quite high. But, with large cats, it's within the realm of possibility to have levels like this (especially if just one cat regurgitates a partly digested meal).
Unfortunately, physiologically toxic levels of nitrite goes unseen right up to the point of no return (brown-blood disease) on large cat species. You'll think your water changes are sufficient mainly because "I've done them the same way for years and they've been fine". But, the fact is the maintenance has to increase parallel to the rate of growth of these cats.
