Light Induced Temperature Change in a 90g Tank: An Example
I haven't looked at my AC Jr's data logs lately, as luckily I've had no reason to. Today, however, I decided to make some modifications on the seasonal temperature program, inspired in part by Craigthor's thread regarding photoperiod; I want my tank's lights to be on at night, even if it means they don't come on until noon...but I digress....
A bit of info. may be helpful for folks to have a reference point. I have 216w total T5 light with tek II reflectors sitting over a 90g tank. The lights are enclosed in a large canopy with an open back - no fans are present. One bank of lights (108w) is on for about 9 hours a day, the other comes on for a 3 hour-or-so burst in the mid afternoon. Now back to my point....
So I uploaded the data logs to my computer, and I noticed that my temperature swings dramatically over the course of a day. The document below shows the graphs themselves; the pH shift is obvious (and expected) enough (my pH is on a timer, so the peaks' and valleys' regular intervals correspond logically)...I did my water change on Monday, evidenced by the severe irregularities in the graph.
The temperature change, is, well, I suppose a bit surprising. My goal is to keep the temp. around 79.8 F; this obviously isn't working out too well, as the only time the temp. is that low is at night, when the lights are off. I increased the "noon burst" this week, from 2.5-3.5 hours, which you can see as a corresponding increase in max. temp over the later part of the week.
What this all means is that the temperature in my tank fluctuates by as much as 5 degrees on some days!! My fish and shrimp so no signs of stress or ill health (some cardinals, a bunch of T. heteromorpha, some oto's, a big bunch of amano shrimp), and behave just as normal at night as they do during the day...though no doubt continued extremes in temperatures can't be good for long term health and survivability.
So...here's the challenge now: finding a way to remedy the problem. I think the easiest (and most painless/cost efficient) method will be to invest in a good quiet fan for the hood. I have faith in evaporative cooling, but don't have too much faith in my fish not catapulting themselves out of the tank...maybe it's time to invest in a hard plastic mesh top as well.
As I finish this, though, I find myself wondering if going and messing around with the tank is going to do more harm than good. I mean, if it ain't broke, don't fix it....right?
Furthermore, I can't help but speculate that this problem is much more common than people probably care to admit (or even know about). Unless you're doing some pretty intensive temperature control (ie chiller), I would wager that those of us using high(er) intensity lighting do see fairly drastic fluctuations in day/nighttime temperatures. Since most of us don't pay that much attention to min/max temps (ie looking at the temp. right before your lights turn off in the evening or right before they come on in the morning), however, it probably just goes unnoticed. For those with more sensitive fish...well, maybe you're more aware.
*notice the "how to capture a screen shot" tab open in the pic...when will they make some aquarium monitors that are mac compatible?...I feel totally incompetent using a PC....