Opinions

Personally I would go with a deep sand bed over a bare bottom tank any day. A bare bottom tank in my opinion just isn't natural looking. Also a deep sand bed with the right grain size will support a ton of micro fauna that will not only help feed you fish but will also help clean the sand as well. For a running system, you really are left with only putting live sand back in. If you do swap it out with livesand I wouldn't hesitate to remove all the crushed coral and add the new live sand all in one go. Just be prepared to have some good quality activated carbon, phosphate remover media and enough water mixed up for a decent water change right after you do it. I also wouldn't attempt swapping out the substrate without having a decent protein skimmer either. That canister filter is likely a major contributor to your nitrate problem as well. Ya figure all the fish waste and excess fish food/nutrients that get sucked into that filter are staying in there, rotting and only adding more nitrates and phosphates to the water. The crushed coral substrate is another nitrate source as well. Crushed coral and other large particle type substrates don't pack together tightly enough to allow for the anoxic area for denitrifying bacteria to live and process the solid waste. As far as sumps go, they are really just an open version of a canister filter. There's not much difference. Water flows down from the display tank into a resivoir under the display and then gets pumped back up to the display. Most times a sump can be as simple as a secondary glass tank with a single or multiple glass or acrylic baffles where the overflow dumps water, the protein skimmer and heater are kept and where the return pump is kept. It's all very simple and the best way to go. That said, even a hang on back style of protein skimmer would be better than the canister filter you have now. By the way, where are you located, maybe someone on here is close enough to lend a hand in helping you get this all sorted out?

1. Another vote for DSB both for function and aesthetics. Some folks like the look of a bare bottom, but regardless you are going to have sinking detritus that will collect in places making it look dirty. With a DSB you won't notice that detritus and you can stock it with creatures that live in the bed to help eat and process the detritus, much more natural. Furthermore a DSB may allow you to keep your current filtration equipment. If you went with a bare bottom I would highly recommend upgrading your filtration equipment. I don't believe that you have to ONLY get live sand. You can probably get away with probably half of the bags being dry sand (cheaper) and the other half live. But try to match the sizes as much as possible and put the dry sand down first then lay the live sand on top. The bacteria in the live sand will spread to the dry sand. If you put the live sand down first you could suffocate the bacteria, killing it and defeating the purpose. The anoxic bacteria will eventually develop in the deepest parts of the sand band. You want a fine sand to accomplish this. DO NOT USE CRUSHED CORAL AS OR BASE OR OTHERWISE. Crushed coral will trap unwanted detritus and allow oxygen into areas where you don't want oxygen. The goal of a DSB is to create a zone at the bottom of the sand bed that oxygen can't get to. They only way to accomplish this is with densely packed fine sand. Some people find in high flow areas the fine sands blows around, so they place and inch or so of slightly larger sand/tiny crushed coral on top of the fine stuff to hold it down. I personally don't like that look and like watching "the dunes" move around. I purchased bags of Nature's Ocean Samoa Pink Sand from Petco.com as my dry sand bags and Caribsea Fiji Pink Aragalive Sand as my live sand. They mixed very well together. Nature's Ocean Aragonite Sand (Same as Samoa Pink, but without the pink particles) is on sale at Petco right now for a pretty good price, plus free shipping check it out www.petco.com/shop/en/petcostore/product/fish/fish-tank-sand-gravel-substrate/natures-ocean-aragonite-sand#

2. Generally canisters are shunned in the salt water world, but they don't have to be. It all depends on how you run and care for them. I run a canister on my tank, and frankly only touch it 3-4 times a year. The key to running a good canister that you don't want to clean out all the time, is to set it up with media that will NOT collect gunk. While this seems like it defeats the purpose of a filter, in a saltwater tank your rocks and DSB are you true filters for the gunk. Anything that gunks up a canister will slow the flow and break down into unwanted nitrates and deliver them back to the tank. A canister won't provide and environment that can break down the nitrates so the process stops there, but the DBS can! Take out the filter pad and other crap, and put crushed rock, or other synthetic bacterial medias like matrix that won't clog and don't need to be changed. Also nix the carbon if you don't plan to change it out on a regular basis, as it becomes saturated and could then recontaminate. My canister basically operates as power head with a biological boost! I only have Pond Matrix in it. It doesn't clog, helps provide extra flow in my tank, and a little more surface area for bacteria. Occasionally I can switch out the media to polish the water for a few days and then swap back to the Matrix for the long term. However, with all that being said, you still should consider something else to pair it with such as a HOB protein skimmer. Personally I don't run a protein skimmer, and have been fine so far. But I have large HOB refugium growing marco algae, and it grows and grows like crazy!

You can turn a 75 gallon into a custom sump for about $250 including the tank substrate, baffles and lighting. Im with Ryan though dsb over bb anyday especially for such a big tank ;0)