Cycling??

So I have been testing my water for the cycle for a couple of weeks. I took out the decomposing shrimp thats been sitting there for a week and a half. Now I tested my water and it measured:
Ammonia 0
Nitrite .05
Nitrate 5

My question though is five days ago my test had nitrate at 10-20 why did they drop by so much. I havent done anything except remove the shrimp and added rodi water for the evaporation.

 

Could be that if you have Live Rock it's been populated with de-nitrifying bacteria. That's good.

 

Some nitrates leave the system as a gas (nitrogen), this is why it is good to have some surface agitation. The nitrogen cycle can be a little confusing. And the types of bacteria that break down the by products of metabolism is confusing as well. Basically animals produce ammonia as a by product of metabolism or decay. Nutrifying bacteria on the surface (areobic) and within the LR and sandbed (anaerobic) break down the ammonia ( extremely toxic) to nitrites ( toxic) to the final product nitrates. Nitrates are removed by water changes and gas exchange. This is a very simplified example as most people really do not care about the specifics of denitrification. Hope it helps a little.

 

How much of a water change would you recommend, Corailline? My ammonia & nitrites have been gone for at least a week, but my nitrates are not going down at all (almost off the chart I have). Have a lot of surface agitation and added chaeto last Saturday (which is growing and the pods in it are having a merry time in there). I'd say between the tank and the sump we're at at least 75 gallons of water overall. We did a 5 gallon "change" Sunday where we pulled 5 gallons of salt & replaced it with 5 gallons of RO to get the salinity to 1.025 (it was between 1.026 & 1.027). The nitrates didn't go down at all after that. There's no livestock in the tank yet other than the pods.