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Water Exchange, Additives or Both? Part I
Water Exchange / Additives - II

In the recent past we published some articles dealing with the problem of reef aquarium maintenance with regard to water exchange and the replenishment of minor and trace elements. Due to the abundance of consequent questions and discussions in reefkeeping forums, we decided to unify and complete the original articles, and address the problem in greater detail.


Version 2012-III

Water exchange, additives or both?

Introduction

In the recent past we published some articles dealing with the problem of reef aquarium maintenance with regard to water exchange and the replenishment of minor and trace elements. Due to the abundance of consequent questions and discussions in reefkeeping forums, we decided to unify and complete the original articles, and address the problem in greater detail.

With regard to the replenishment of depleted minor and trace elements, the reefkeeping community can be divided into the following groups

(For all groups parallel replenishment of Ca, Mg and Carbonates is assumed)
Water Exchange Use of Additives
Yes
No
Regular
Majority
Minority
None
Minimum
-

Even though reef tanks operating with no water exchange do exist, their number is negligible; we will therefore concentrate in this article only on those aquarists, regularly exchanging a certain amount of water. In this group, two diametrically opposite approaches exist, which could in a simplified manner be defined as follows:

  • Water exchange + use of additives
  • Only water exchange
    • the replenishment of depleted minor and trace elements
      is achieved through their increased concentration in the used salt

Types of Marine Salts

The large discrepancy between aforementioned approaches is not the only problem. Additional one is namely the existence of two different types of marine salt – one with a natural and one with an increased concentration of minor and trace elements. It is quite certain, that a lot of reefkeepers after having read these lines will realize that they have never given too much thought about this fact.

Based on this the following cases can occur:

Water Exchange
Type of used salt based on the concentration
of minor and trace elements
Use of Additives
Naturally concentrated(a)
No
Enriched(b)
No
Naturally concentrated
Yes
Enriched
Yes

(a) The salt contains the correct amount of minor and trace elements, so that after dilution the sea water with a natural concentration of these elements is produced.


(b) The salt contains an up to 100 times higher concentration of minor and trace elements than in natural seawater. After dilution the concentration of minor and trace elements in the produced seawater is about 3 times higher than in natural seawater.

 

Water Exchange

Why do we actually exchange the water? To this question different reefkeepers deliver different answers. Some exchange water in order to remove unwanted matter, mainly the one that cannot be removed by traditional methods (liquid removers, sorbents, skimmers, etc.), others exchange water in order to resupply depleted trace elements and others do not really think about the reasons behind; they perform this task just because it should be done – in a similar way, as people air the room from time to time. All aforementioned answers are to a greater or lesser extent valid.

Water exchange in reef tanks is – similarly to airing a room – an exchange of part of the environment which surrounds the living organisms. In this manner part of the unwanted matter is removed from water (unfortunately however also together with a part of the wanted matter). If the salt used for the production of seawater is of high purity, no unwanted material returns to the system – a decrease in its concentration is achieved and the tank environment is improved. As far as the minor and trace elements are concerned, the situation is not so simple and we will have to dedicate ourselves to this problem further.

Is the regular water exchange with the use of a suitable marine salt enough for the complete long-lasting care of a reef tank, or is it better – perhaps even necessary – to use additives?

Starting Points

In order to be able to give an answer to the previous question, first of all the basic starting points for further reflections:

  • The measurement of the actual concentrations of the majority of minor and trace elements in common aquaristics practice is infeasible.
  • The concentration of minor and trace elements in reef tanks decreases steadily
    from following reasons:
    • Biological processes (metabolism of living organisms)
    • Physicochemical processes (absorption on sorbents,
      escaping through skimming and water evaporation)
    • Various unspecified chemical reactions in the system
  • The depletion rate of each element may vary greatly - from single to double-digit percentages.
  • The long-term development of the concentration of minor and trace elements in the tank is mainly influenced by
    • the water exchange extent and frequency
    • the type of salt used for the seawater make-up
    • depletion rate for each element

Extreme Cases

Let’s assume a hypothetical trace element X, with a 15% monthly depletion rate; regarding water exchange, two marginal cases may occur:

0% water exchange. 100% water exchange.
No water exchange whatsoever takes place. The constant decline of the element X concentration illustrated in the diagram on the left should therefore come as no surprise to anyone. After 24 months the concentration of the element X reaches 10% of its natural value and converges infinitely to zero.
The entire volume of the tank is exchanged monthly. Used salt contains the element X in such concentrations, that the replenishment exactly meets its depletion. In this case the concentration of the element X fluctuates slightly at each water exchange interval and is basically constant and correct.
Limit 1
Limit 2

Extreme cases are of course very rare in reality. As already mentioned, tanks operated long-term without water exchange are an exception, and a 100% water change is - not only due to financial reasons - absolutely unrealistic. Furthermore reef tanks contain tens of important elements in the different concentrations and with different depletion rates.

Analysis

Parameters

In order to simplify the matters and for better understanding we will introduce a case study, in which the default parameters will be set as follows:

Tank volume no infuence
Water exchange Frequency 1 x monthly
Amount 10%
Types of salt
for seawater make-up
1) Natural concentration of minor and trace elements
2) Hundredfold of natural minor and trace elements concentration
Monitored
elements
1 05% monthly depletion
2 25% monthly depletion
3 50% monthly depletion

Diagram explanation

  • X-axis - shows the time in months, starting at a point where the concentration of all monitored elements was correct - i.e. corresponding to natural or suggested levels.
  • Y-axis - a value of 100% indicates a correct concentration of the given trace element i.e. levels corresponding to natural or suggested concentrations.
  • The displayed curves with the concentration profile of each element have been smoothed out, without the fluctuations occurring at each water exchange interval.
 

Naturally Concentrated Salt – No Additives

The marine salt contains such amounts of inorganic compounds, ensuring after dilution an exact copy of natural seawater with correct concentration of all trace elements. No additives are used for the simultaneous replenishment of trace elements.

02

After ca 18 months a steady-state is reached, in which a one-sided “minus” ion imbalance occurs. All monitored elements occurse in the tank under their natural or suggested concentration levels.

Enriched Salt – No Additives

Marine salt contains up to 100 times higher concentration of trace elements than natural seawater. After dilution the produced water contains ca 3,3 times more trace elements than natural seawater. No additives for the replenishment of trace elements are used.

03

After ca 18 months a steady-state is reached, in which a two-sided “plus-minus” ion imbalance occurs. Monitored elements occurse in the tank both under and above their natural or suggested concentration levels.

Enriched Salt – Use of Additives

Marine salt contains up to 100 times higher concentration of trace elements than natural seawater. After dilution the produced water contains ca 3,3 times more trace elements than natural seawater. The additives for replenishment of depleted trace elements are used in parallel.

03

After ca 18 months a steady-state is reached, in which a one-sided “plus” ion imbalance occurs. High dosage, especially of mixed additives, may however lead to a continuous concentration increase of certain elements. The majority of elements will occur in the tank in significantly higher concentrations than in natural seawater.

Naturally Concentrated Salt – Use of Additives

The marine salt contains such amounts of inorganic compounds, ensuring after dilution an exact copy of natural seawater with correct concentration of all trace elements. The additives for replenishment of depleted trace elements are used in parallel.

04

The element concentration is controlled by the additives and is constant on a long-term basis, approximating natural concentration levels. With the right approach to choosing and applying additives, the problem of elements depletion and their subsequent low concentration is solved, while at the same time the risk of exceeding natural concentrations is minimal.

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