i actually noticed that others had to water changes for up to 2 months! the cement i used was a different from what others generally use. i noticed that most would use the 40lb bags of cement that are for general use, while i used a 10lb bucket of hydrolic comercial grade cement that is used for patch work, like for patching swimming pools, and water fountains. its says that its water proof, which to me suggested that it wouldnt dissolve slowly and effect the water. maybe this type of cement is the key to making a diy background. when i first filled the tank around 24-36 hrs the ph was very high around 6 hrs after filling. so i drained it and let it dry out completly until the next day. i then filled and drined it 2 more times adding aquarium salt to it each time. the ph was back to normal in 2 days. i added the wood and fish back in and they are all still doing great! i think the trick to getting it to cure was waiting 48 hrs befor it touched the water, although mine was filled after one day i curing. 48hrs with the cement i used, seemed to do the trick. aquarium salt also helps i hear,thats why i had added it.