02-21-2010, 12:59 PM
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Three years is what Diana Walstad suggests as viable life for a T8, and I have read the same elsewhere--but not everyone holds this view. I have no scientific data either way. Plants are not likely to up and die on the day following the end of year 1 or year 3, whichever, so I would suggest observation. Once a planted aquarium is biological established and allowed to run itself, in a manner of speaking, things remain stable. If the plants after several months or 1 or 2 years begin to show obvious issues, at that point I would consider the light as a probable cause, assuming I had maintained fertilization, fish load, water changes basically the same all along.
I know I keep harping on balance, but it really is the crucial issue for a couple of aspects in planted tanks. Once that balance is established--and it can't occur with constant fiddling over light changes, adding Excel now and then, changing fertilization routines significantly--it will tend to stay. My Echinodorus macrophyllus that is now more than 12 years old would not be in the tank today if it had not been in a stable environment with regular consistent fertilization and water changes.