Are you sure you don't want to talk about a less sensitive subject like religion or politics?
In my opinion water changes are good for the following purposes, in order of importance/effectiveness.
- removing detritus
- reducing/diluting secondary metabolites (algae & coral toxins)
- reducing/diluting heavy metals, or what we call trace elements in the aquarium hobby
- reducing/diluting vitamins
- reducing/diluting nitrogen (ammonia, nitrite, nitrate)
- reducing/diluting phosphate
- reducing/diluting bacteria
- reducing/diluting TOC
Water changes are limited by the percentage you exchange. A 10% water change removes 10% of the "bad stuff". The exception to this is detritus removal, if you are vacuuming the substrate, blasting out rock work and vacuuming the sump. Water changes do not effectively replenish water chemistry, as it is limited by the same percentage issue. In other words, a 10% water change only assures that 10% of the total water volume has the right proportions of calcium, carbonates, magnesium, and all the other "good stuff". While it is nice to remove some of the bad stuff, filtration devices are more efficient and calcium reactors and chemical dosing assures that 100% of the water has the proper water chemistry (good stuff).
Water changes can cause harm if they aren't carried out diligently. Some of the negative aspects of water changes are...
- reduction of pro-biotics (bacteria & plankton)
- introduction of impurities via source water, salt mix, mixing tools or hose
- introduction of excess trace elements and vitamins from salt mix
- temperature fluctuation
- salinity fluctuation
- PH, KH, calcium, magnesium etc. shock from bad salt mix
- classified/non-homogenous salt mix due to partial bucket or bag use
- old, clumped/compromised salt mix
- exposure of corals to atmospheric air
- partially dissolved salt mix
- poorly aerated salt mix
- accidental overfilling system
- accidentally over-draining system
- sand bed disturbance releasing hydrogen sulphide or depleting DSB infauna (beneficial organisms)
The main issue with water changes is they need to be calibrated to the demand. If you have "x" amount of nutrients building up in your system, then you need to do water changes according to that demand. A 10% water change will reduce your 20ppm nitrate down to 18ppm, but your residual nitrate accumulation may be at a faster rate than your weekly or monthly water changes. We aren't talking about a static amount that you can slowly chip away at, unless you have filtration devices and nutrient export of other sorts to make up the difference. If that is the case, water changes may not be necessary, and they are clearly the most expensive and least effective method of nutrient export.
We know that zero nitrates and phosphates can be maintained without water changes through carbon dosing, DSB, GFO and refugia to name a few. We also know that water chemistry can be maintained without water changes, and that there is an excess not a deficit of trace elements. Why add trace elements (heavy metals) when we statistically have too many? Most reef tanks don't require physical removal of detritus, including many of the tanks that receive major and frequent water changes. This only leaves secondary metabolites as an agent that we need to export. It is possible that this is enough justification for water changes, but it is equally possible that they are removed more efficiently through UV, ozone, protein skimming, mechanical filtration, mangrove trees, macro algae, carbon, bacterial assimilation, biological assimilation by micro organisms and coral, or simply time.
In evaluating any procedure you must first establish what you are trying to accomplish and why you are doing so. If water changes offer something that you are not getting with your current regimen, and you feel there is a demand in the first place, then by all means do so. On the other hand, if you feel that your application has all of these criteria covered and see no need for adding trace elements & vitamins, then water changes may no be a cost effective method of maintaing your reef.
The bigger the tank, the less you rely on water changes, and vice versa. A reef tank of 50 gallon or less, can be maintained with major weekly water changes at a lower cost than purchasing UV, ozone, calcium reactor, dosing systems, a refugium and mechanical filter. You can reinvest the capital and operational costs into a good source water filter, salt, and a water changing system. Once you get over 200 gallons, water changes are less appealing.
This doesn't mean you can stop doing water changes without consequence. Many people claim that their tanks look better after water changes. If you have a good system and are confident that it can be somewhat self sustaining, then slowly reduce water change frequency or volume. If you see negative repercussions, then resume water changes as before.
