Ok, a lot going on here that can contribute to a crashed tank. Lets see if I can get you on the right path. If you don't have funds to correct the issue, you have all the equipment you need already to have a nice freshwater planted tank without the expense of a marine tank. Just a suggestion.
1. You are using dechlorinated tap water. This is a no-no when it comes to marine systems. Conditioned tap water will contain other chemicals in it that are not neutralized with a dechlorinator. Some tap water even contains trace amounts of ammonia. Only use RO/DI purified water to mix your salt in, and only use RO/DI water with a TDS of 0ppm to top off with. It looks like you need to top off your tank as well, immediately.
2. Double check your salinity and make sure it's not rising/falling too rapidly. If you are using a hydrometer, check your reading against a water sample you take to your LFS that checks it with their refractometer. In the future, buy and consistently use a calibrated refractometer when measuring your salinity. Check it weekly and check your tanks salinity and your water mix salinity every single time before a water change. Make sure your tank is not leaking.
3. Do not use canister filters on a saltwater tank. It's a really bad idea. They become clogged without regular maintenance every two weeks to thirty days. Anaerobic bacteria eventually form isolated pockets of hydrogen sulfide, which is what you were smelling in your water earlier - that weird almost rotten egg smell... in that case, a pocket of hydrogen sulfide was more than likely stirred up somewhere in your system.
4. An aquipure denitrate filter is not needed, especially when you have such a small bioload and the ability to do water changes as needed. It's an extra expense, however, it is justifiable if that is an excuse to keep vodka in the freezer.
5. Take off any glass tops on top of the tank. Glass tops are mainly for freshwater systems where you want to increase the amount of CO2 in the system to lower the pH. In marine systems, we try to keep as much CO2 out of the tank as possible - thereby increasing surface area through sumps, skimmers, scrubbers, and massive amounts of water flow... which leads me to number 6.
6. Two maxijet 600 powerheads are not enough flow for a 55g tank, which is what your tank appears to be to me. That's roughly 150gph each coming out of the nozzles - and that's only a 6x turnover rate. You do not count turnover rate coming off of return pipes like your canister filters. One of the main reasons why your pH is low is because there isn't enough water flow and more than likely you have glass tops on the tank. Get upgraded powerheads. Koralia brand powerheads are cheap and effective. There are also some cheap aqueon or even oceanic powerheads that are all kind of shaped the same - with a propeller and a broad flow pattern. Use these types of powerheads to push water across the surface of the water and down and around your rocks.
7. Since I don't recommend using canister filters, I highly suggest investing in a HOB filter or two that will really work for your system. Marineland Penguin filters, Marineland Emperor filters, and even Aquaclear filters are great brands that would do well for your system. I recommend getting two that are rated for your size tank each. Preferably, get a filter system with a basket, so that you can add additional chemical filtration when needed.
8. Now that a lot of things have died in your tank, you should remove everything that is dead and monitor your ammonia and nitrite levels until then return back to zero, which I doubt they will until you get a proper source of top off water.
9. I personally don't feel like you have enough rock for the system to be healthy either. Adding even some base rock to build your rock work higher up will add additional biofiltration that will help curb the potential for another crash once it becomes established.
10. I don't feel like you are using food that is nutritious enough for long term care of the fish. Using frozen foods or freshly prepared frozen DIY foods is a great start to healthy fish and a healthy tank. Also, a mix of different types of flake foods can also be much better then the freeze dried junk that you are feeding now. I realize that you probably didn't realize this when you purchased the food, but now you know.
A set of questions for you, and then I'll give you additional information. Once you have responded to them, please go ahead and private message me again. I don't come to these forums as often as I used to, so things get lost really easily.
1. What test kits are you using?
2. How long has the tank been set up?
3. What is the extent of your experience with saltwater systems?
4. What books have you read about saltwater aquariums?
5. What has inspired you do get into the hobby?
6. Are there any chemicals you are currently using in your system?
7. Are you burning incense, smoking, have any air fresheners, roach, ant or other bug spray around the tank?
8. How often are you servicing your canister filters? What media does it contain?
9. Do you have a digital thermometer? How consistent is your temperature staying? (our tanks like consistency, so fluctuating temps can cause mass death.)
10. Do you have the drive and the financial backing to make these changes? If not, I highly suggest taking the equipment you have now and getting into a much cheaper hobby with freshwater planted tanks. Saltwater tanks can be a huge money pit if you don't know what you are doing. I know this is more of a statement then a question, I apologize.
11. More up close pictures of your setup would be nice. Take pictures of your system before and after any of the changes I have recommended, please.
If you listen to my suggestions and follow (most) of them and listen to others advice and take everything with a grain of salt, I'm pretty sure you will become a successful marine aquarist.
