A word of warning. The day after changing my filter three months ago fish started dying. Next morning my 6″ long Plecostomus was dead. Then three Clown Loaches, each about 5 ” long. Within a week I had lost 18 of my 20 fish. Yet there was no sign of disease.

A water test and much internet searching finally pointed out what I had done wrong. I had neglected my tank for a few weeks. In a rush to catch up, I siphoned out about 30 percent of the water (more than usual), did a “better” than usual job of siphoning up detritus from the gravel. Then – my biggest mistake – I replaced the Penguin filter too! The result is the water chemistry changed too quickly. The Nitrates spiked to a deadly level. My tank, healthy for more than two years, needed to be started over. Took two months and many water changes before I could safely add new fish.

I visited perhaps a dozen “fish forums” and only one had a comment saying “don’t change filters when you do a water change.’ The Penguin filters and the box they come in carry no warnings. What I (and you) should do is simple: space your water changes and filter changes a week or two apart.

Now knowing how important the “aged” filter is (especially in a tank like mine with no under gravel filter) I’ve started this habit: there’s just enough space to get a second filter flat against the old filter. After a couple of weeks, I remove the older filter and put the newer one – by now doing some biological filtering – in its place. That way the tank is never depending on only a new filter. For larger tanks (say 55 gal and up), the 330 or 350 Penguin pump holds two filters side by side, so just change one filter at a time.

Hope this helps you avoid the disaster that hit my tank
Rating: 5 / 5

Funny, I changed all 8 at the same time and threw my tank out of whack. Too much algae and goo left the system with the old filters. I’ll change ‘em in stages from now on.
Rating: 5 / 5