Cleaning freshwater tank

Question
Hi Joel
I have a 207ltr freshwater tank that has been operational for 5 months; everything has been great up until Xmas Eve! I performed a 25% water change and gravel vacuumed. I tested the water on Xmas day for ammonia and nitrite both showed 0.
On Wednesday I added a piece of  bogwood with a plant attached to it along with a bristle nose (both birthday pressies) the bristle nose just sits there and doesn't hardly do anything so I tested the water ammonia showed 0 but my Nitrite showed 0.1 plus I lost a female guppy just ready to give birth. I panicked and done a 50% water change and added JBL Denitrol along with correct amount of Aqua One water conditioner.
After testing the water last night and this morning the Ammonia still reads 0 but the Nitrite has only dropped very slightly.
I am very confused as to how this has happened.
Should I perform another water change today? I have added some more JBL Denitrol but very little difference.
When I cleaned the tank on Xmas Eve I only very gentley gravel vacuumed but lifted all the rocks/ornaments and a very light clean of the filter in tank water just to remove some weed that was stuck in it. (Otto internal power filter 800/l)
Could the bogwood that I added have caused this?
Any info would help.
Many Thanks
Faye
Ps Ph is 7 tank is buffered using shell
Tap water is ph 7 carbon was replaced in filter appox a month ago  

Answer
Dear Faye
Yes the problem probably was the bogwood you have to boil the bogwood before you put it in eventhough the petshop says it has aready been done.  Your bristle nose is sposed to just sit there he is a catfish moving a lot isnt really their thing