Ammonia After Filter Change

[Jonesthecatfish]

Ive had a cycled 95 litre tank with 15 fish . Unfortunately my internal filter began to stop working 6 months into its 2 year guarantee . My LFS replaced it with a different and more powerful filter and my old filter media was in tank water for 2 hours until i got it into the new filter. I dosed the new filter with Bacterlife to replace any bacteria that would have died off in the old media during the 2 hours the tank was was without a filter, Now ive got Ammonia of 1. Nitrites 0 , Nitrates 5 , PH 7 . Previously before the old filter packed up i had Ammonia 0 , Nitrites 0 , Nitrates 40 ( tap water 20) , PH 7 . I havent fed the fish for 2 days . Did a 25 % water change and used Ammonia remover which gets it down but after 12 hours its back up to 1 again. Could the few very small dieing leaves at the back of the tank be causing it ? I was going to wait till they completely died as they were there before when the readings were ok .