Thanks for the reply. Some suggestions:
01. Don't use the aquarium salt. If it isn't a Rift-Lake specific item, it is mostly NaCl (sodium chloride). The Rift Lakes have almost no sodium chloride in them. The Seachem Rift Lake salts provide the correct trace elements, and the buffer is almost the only thing that will raise the pH to where the fish really prefer it to be. The Seachem products are available in 1 kg containers from any of the major on-line retailers, and will cost you less than $25 for a year's supply.
02. In your canister, lose the carbon, not the biomax. Carbon is only effective at adsorption for the first 48 hours or so. Otherwise, you'd be better off using bioballs or other biomedia (RTR particularly likes the Dupla mini-kaskade for this purpose, and I'd have to say I agree with him).
03. If you want to add carbon periodically, put a pack in your Aquaclear 500, and then take it out after a couple of days. You will thus get 100% of the carbon benefit without taking up permanent space in your filter. If you want to put a media bag of crushed coral on top of your sponge in the AC500, you will get the buffering benefits (this would mean that you wouldn't need to use as much of the Seachem buffering product, although it wouldn't really affect how much of the Seachem Rift Lake Salts you should be using). BTW, if your sponge in the AC500 is trying to float, it is clogged and needs to be vigorously rinsed/squeezed out in a bucket or two of tank water to get rid of the detritus (accumulated organics are bad anyway) and restore the flow (for better bio filtration).
04. Regarding the frozen food: most sources recommend that you thaw it before feeding. Just throw it in a container with some tank water, let it unfreeze, and then pour it in. It won't float anymore, and perhaps your fish in distress will recover from (a) not getting internal freezer burn and (2) not gulping air with the floating food.
Hope this helps. Good luck.