The crown will grow back once or twice, but it is a sign of starvation. They need quite a lot of suspended food to survive, and most tanks don't have enough concentrations for them to survive for more than a few months. Target feeding near them with plankton/filter feeder food when the crown is open may help them survive.
I have a deltec MCE500 which slots nicely into the first chamber in the sump (I have the weir to the right). The return pump is an Eheim Compact+ 5000. I've also run it with an Eheim Compact+ 3000 but the head is very high and the return not as strong as i wanted with the 3000.
I recommend ND aquatics: http://www.ndaquatics.co.uk/ You get the works from them: tank, stand and sump (and they can provide the plumbing as well), and they have a lot of other equipment as well. I bought my tank from them last year and have not looked back!
I'd up the pump - the flow should be strong enough to just keep the media moving. Someone on the forom said it should look like "something is crawling under the media surface". It's possible that the media gets clogged over time, so if you put a valve on the inlet you can always run a more powerful pump and adjust the flow rate over time.
I'm sorry to hear about your losses - it is an awful feeling to bring new fishes home onyl to see them perish.
I think there might be two problems with your system. First, you don't quarantine the fish (and since one of them seem to have an infection of some kind I would not add anything until you're sure that's gone). Secondly, I think you're adding too many too soon - a rule of thumb is 1 fish per month (or pair of clowns etc). This is so the filtration can adjust to the new bioload through a new minicycling. So I'd get a quarantine tank up and running ASAP, move the poorly chromis to that and then wait, wait and wait a bit more. I know it's frustrating, I'm currently upgrading my system and have to hit myself in the head to avoid moving the fish over too soon.
Have you tried true peppermint shrimps? It is worth a shot, although they can be a bit of a hit and miss with aptasias. They worked like a charm for me: I put two of them in my infested 40g tank and the aptasias were gone within a week. That was around christmas time and I haven't seen an aptasia since. Oh, and both shrimps are still around scavenging near the bottom.