Seen these two test displays that sit inside your tank.

Seachem PH Alert and Seachem Ammonia Alert.

Bought both off of Ebay at about £12 for the two. Has anybody got them, used them. They stick inside of the tank and last up to a year.

Quite a good idea actually. However, if you test your water vigorously once a week, you probably don't need them. Having said that, at least it gives you prior warning of something has gone wrong in the middle of water tests.