Tested my water....

Your best bet now is to take water sample to the LFS and have them test it. Take plenty of water.

 

Did you test right after you cleaned and changed the substrate??? If you disturbed the bed, you released alot of nitrates and those numbers would probably be accurate.

 

That's my thought too. Your old substrate was probably housing a lot of the beneficial bacteria.

Take some water to the LFS for sure!

You should do a 50% water change ASAP. Remember to let the water change water heat up to the correct level. Remember to add your de-chlorintaor to it. Remember to match your salinity. If you had more time I would say to let it aerate overnight but you need to do another water change now. You can do a 100% water change everyday f you wanted to, but in reality it is probably harder to match the exact parameters of your existing tank. At the aquarium I went to they did even MORE than a 100% water change daily, it probably equaled about 10 a day. But they had it on a timed system so the tanks performed the water change by themseles.

WATER CHANGE ASAP.

This should solve your problem because I really believe your problem has to do with the new sand. Also, you should buy a new carbon filter cartrdige. Don't take out the old cartridge cuz you need the bacteria on it, but the carbon should help. Also, I went a bought a $20 bag of pebbles that was called "nitrate removal" and put the pebbles in a bag and put it in the HOB filter. I put another bag in the sump of my 75g. I also bought a nitrate removal filter to place above my bioballs. OK, anyways.....

water change asap