We have all heard about new tank syndrome it is a condition which affects a good number of new hobbyists after being given poor advice from a vendor.
But for some reason far less hobbyists are aware of old tank syndrome and it is even more surprising because it probably affects more aquariums than new tank syndrome.
Old tank syndrome as it's name suggests occurs in aquariums which have been set up for a long time or where tank maintenance hasn't kept pace with the processes occurring in the aquarium. When tank maintenance is allowed to lapse the first thing that happens is a rise in the nitrate level. Nitrate is the end product and least toxic compound of the nitrogen cycle. If water changes are allowed to lapse or are of insufficient volume or frequency then nitrates will continue to rise. As mentioned nitrate isn't very toxic to fish but at higher levels it will impact on their health.
Another effect of a lapse in water changes is falling carbonates which will eventually lead to the pH falling to dangerous levels and in turn this will lead to the biological filtration performing very inefficiently since filter bacteria do better in slightly alkaline water with a good carbonate reserve.
Regardless of maintenance bio media with a very high surface area will eventually become clogged up with dead bacteria and will have a much reduced surface area as a result.
So to recap, old tank syndrome causes:
All of which eventually leads to fish with poor growth,and vigour, fish becoming less fertile or even sterile, poor fish health and premature deaths.
Don't rely on a formula for making water changes. The often recommended 25% fortnightly water change may or may not be right for your tank, every tank is different and water change regimes need to reflect this. A lightly stocked well planted tank will need less water changes than a well stocked mbuna tank this much should be obvious to everyone. But how do you know what is right for your tank?
Measure the nitrate level on a regular basis, that's how. If you are making sufficient water changes the nitrate level will be below 20mg/l and it will be fairly stable month after month. If it does rise even just a little then the water changes need to be increased.
There is no maximum or minimum amount when it comes to water changes, contrary to popular opinion you cannot change to much water. It is a big change in water chemistry which could be harmful and distressing to fish this means that if large water changes were made on a frequent basis then the water chemistry would remain fairly stable and such a regime would greatly benefit the fish. On the other hand making large scale water changes on an infrequent basis will result in a big change in water chemistry and this is to be avoided.
Water changes don't just dilute nitrates. Water changes replenish carbonates and important trace elements.
Use a sponge for bio media instead of ceramic or sintered glass, sponge doesn't have the same massive surface area of those materials but it can be easily rinsed clean. Once all the tiny nooks and crannies of sintered glass are full of dead bacteria it loses most of its useful surface area and will need replacing. When replacing this media it should be done in stages and not all at once. Try replacing 50% of it every 6 months which means that the media has an effective life of one year. A sponge although with less surface area can be used indefinitely.
Don't dive in and make a huge water change, even though the water would be very much improved the risk to the fish would be unacceptable. Instead make daily small water changes and gradually bring the nitrate level down to an acceptable level.
Clean the filter, don't be to thorough, as long as there is a good water flow through the filter it will work just fine. If the bio media is sintered glass or ceramic replace some of it with new media, if it is a sponge then a gentle rinse will be sufficient.
In soft water areas add some extra buffer - Kent pH Stable or even good old bicarbonate of soda will do for this.
To begin with feed the fish very lightly, this will help avoid ammonia or nitrite spikes until the biological filtration has had time to recover. After about one month has passed a more normal maintenance regime can be adopted and you will notice the change which has taken place in the tank and its occupants.
Old tank syndrome is easily avoided with good regular maintenance and your aquarium will reflect this.