A question re: filtration, water changes, and Tropheus sp.

[BrownBullhead]

A question re: filtration, water changes, and Tropheus sp.

I feel a li'l sheepish, dumb, whatever, in asking this question, but I am quite curious to see the opinions I will receive, so here goes.

For an "established" (running longer than 6 months) aquarium, 150-gallon, filtered by a Rena Filstar xP3 w/ 2 of 6 basket trays filled with biological media, and also filtered by 3 x Hydro-Sponge #3 sponge filters, my question relates to water changes...

Is it required to maintain 50% weekly water changes in a tank this size (75 gallons per week) when only 16 Tropheus Moorii Nkonde of 3.5" to 4.0" inhabit the setup?

I would like to think I could cheat a little because of the sheer volume of water versus the amount of waste produced by 16 fish. What do you people think?

[MatsP]

The best way to determine water changing regime is to determine what the maximum allowed level(s) of pollutant(s) are.

Let's pick Nitrate, as that's one of the easier to monitor. You could look at other polluting criteria...

So, let's pick a level of nitrate: 20 ppm.

So your goal is to keep the tank under 20 ppm at all times.

How much water you need to replace depends on the level of nitrate, which in turn depends on stocking level (fish numbers and sizes), feeding level (more food -> more pollution)... If we start out with a tank that has 12 ppm of nitrate, and nitrate goes up by 8 ppm per week, then you need to replace (at least) 40% of the water to get back to 12ppm. [Of course, this assumes your tap-water is near zero nitrate - this may or may not be true, and make sure your nitrate test is fresh].

You can calculate the size of the water change by this formula:

change = (1 - (allowed - rise) / allowed) * 100.
allowed is the ppm level you aim to keep.
rise is the amount of ppm's that your water rises from one water change to the next.
Multiply by 100 to get percentage.
The "1 -" at the start is because the calculation is "backwards", it calculates how much of the water you should keep in the tank.

So if we insert the numbers from above example:
20 - 8 = 12 ; allowed - rise
12 / 20 = 0.6 ; (allowed - rise) / allowed
1 - 0.6 = 0.4 ; Now calculate "the other part".
0.4 * 100 = 40 ; Make it into percent.

Now for some further brain-excercise: Instead of multiplying by 100, multiply by the water-depth in the tank, say 15": 0.4 * 15 = 6.0. So to do 40% water change you drain 6 inches of water...

The real difficulty with the above calculation is to know the rise level with enough priecision. Most nitrate test kits are too inprecise to give a good measure. They usually only show difference between 12ppm and 25ppm and 50ppm, for example.

My personal opinion would be that 50% per week is a bit excessive with the amount of fish you have. But I've got no experience with those types of fish.

--
Mats

[BrownBullhead]

Thank you for the concise input. I am still trying to get this colony to breed and periodically they go through the motions after the large water changes, so I may keep it up for a few more weeks and see if I can finaly induce some breeding. I attempted to "vent" them during my last MAJOR maintenance (when I had to remove them from the tank) and it appears I have 9 males and 7 females, so I hope something will happen eventually. As I said I had never "vented" fish before so let's hope I was correct? They now have the tank to themselves as I removed my 24+ Syno. Petricola... mind you, a colony of Tropheus Black "Kiriza" that lived in the tank earlier this year with the Petricola didn't seem to mind and were breeding quite well until I had some maintenance issues and a bunch of Tropheus fry died off. These Nkonde are being super-finicky for me and I don't know why... the Kiriza were so "easy." It doesn't help ease my mind much when other Nkonde keepers on the forums are breeding them as easy as Kiriza bred for me.

[Ken Boorman]

This is probably the best reference I know for showing you how to vent fish:

http://www.fishhead.com/articles/ventsex.htm

Hope that helps someone

Ken

[BrownBullhead]

Rhad: Thanks for the link! I had used the one Tropheus genitalia picture in my previous attempt... will need to study the "mouth" image more closely! Cheers!