Some weird things happening in my tank. 2 new fish dead

My tank has been running 10 months now. I recently did a 50% water change and lightly vacuumed the sand bed of the tank as it was looking really dirty like a layer of brownish sand on the top as it had been a little while since my last cleaning. I also completely cleaned out my canister filter as it had been about 4 months since i last cleaned it. I let the water settle 3 days after the change and it was crystal clear water and pearly white sand and tests indicated a health living environment . I only have 4 small fish in the 55 gal tank as well as a shrimp and a crab and a snail. So i decided to buy a small spotted sweet lips and a fire gobby. I drip acclimated the fish for an hour and half then introduced them to the new environment. With in 2 days and with no signs of ill health both new fish died. i bought the fish from a store that i normally don't go through so that may be a factor. it was so weird like i fed them they ate and had tons of life in them and then i turned the light off and the next morning poof both dead. All other life in the tank is fine. now i am 7 days from when i did the 50% water change and my sand is even more brown than before and has never gotten so brown so fast and nitrite is at .20 pmm when i have kept it at 0 for the better half of 10 months. I have no idea what is going on and I am scared of having a tank crash any help or advice would really help. i have never seen it get so dirty so fast and haven't lost a fish since the first month it has been up

 

When you cleaned out your canister filter, you likely killed off a lot of the beneficial bacteria that helps keep your tank in check. Next time you do that, rinse the filter and filter media with salt water and try to limit it's exposure to air while you're cleaning it.

 

+1 ^ also the combo of stirring up toxins in sand bed and killing off you bacteria probably did your little guys in;(.Hold off fish until water parameters stabilize Have you considered a sump??

 

No one finds it odd that all the life in my tank is still alive and the 2 new fish both died on the same day without any warning?

 

Tank inhabitants can build a tolerance to less than ideal or slowly changing swings in water quality. New inhabitants trying to acclimate can fall victim to dramatic differences in quality from where they originated.

The presence of Nitrites and persistent brown to the sand in a tank that age sounds like it should be the major concern.

I agree with the others, big cleanings of sand and canisters can cause big problems. Smaller doses of tank maintenance are better.

Please post an image of the brown sand and maybe we can get an ID.

 

A little OT, and you may already know this, but Spotted Sweet Lips get huge! Literally over half the length of the 55. Adults should be in tanks 10 times as large.

 

I think that your cleaning the canister after such a long time had to mess with your set up's stability and the new additions fell victim while your current fish who were adjusted to your tank were able to handle it because they were not stressed (which all new fish are stressed to some degree). I'm sorry for your loss

 

Here is a picture of my sand bed. I want this **** gone!!!!! it is such an eye sore i don't even like looking at my tank. can someone ID this and tell me how to get my sand white again......I don't understand it has never ever gotten this bad and it has only been a week since the cleaning

 

wow, that is quite an intense amount - do you run carbon or any phosphate absorbing media? It almost looks like the color of dinos but I don't see any bubbles in it. I'll wait for others to chime in......

 

yes i do run carbon. I check my phosphates they are low. I don't get it. All my old critters seem fine. eating well sleeping normally no weird activity. I just don't understand whats happening it's stressful and aggravating. My sand has never been this bad and it has only been a week since i cleaned the tank. How do i get rid of it??