Ok here is what happened. I have a 150 gallon aquarium with frontosa I fed them some shrimp as I do from time to time I saw that all the pieces I fed were eaten so I went to bed. Woke up the next morning and saw a piece of shrimp floating around (I assume one of the frontosa spit it out) removed the shrimp tested the water and my nitrites were 15 ppm I thought it had to be a false reading so I used my back up api master kit and got same reading so of course I went into full panic mode did 75% water change 4 times back to back rechecked it and everything is good except my nitrates are 0 and there is a smell that to me smells like garlic or sulpherish and the water has a slight brownish tint to it when looking from the side now which it did not have before. I habe no wood in tge aquarium so tanins are out. Any recommendation or tips on what I should do. I run 3 fluval fx6 and an ehiem 2262 fluval marine reef led light and a beamworks led light. Jim silicate pool filter sand substrate and feed northern and nls pellets. Hope you have all the info you need to help. Thanks