We went to the aquarium and got something we ought to have got right at the start: an external canister filter. We went with a Fluval 303 (I think) – we could have got a smaller one, but that would have just been big enough for our tank. As it is, this one cycles the entire tank’s worth of water something like 5 or 6 times an hour.

In 24 hours, we noticed an incredible change: the algal bloom (which we assumed was an algal bloom, but may only have been dirty water!) disappeared, and within three days we could see from one end to the other and everywhere else. The filter has a carbon layer which gets all the particulate matter out of the water, along with the sponges, and a section with a biological-filter-thing: it’s got lots of tiny holes in it, so there’s a great deal of surface area, so a lot of bacteria grows. Which would be good, given that our nitrites are incredibly high. Which would probably account for the death of another platy.

I can’t believe we didn’t get a filter originally, that the shop didn’t tell us to get one. It makes no sense! It provides an enormous area for bacteria to grow, and that in turn must obviously hasten the ammonia cycling process.

We’ve just been reading about fishless cycling. I’m not sure whether you add plants, but you do add ammonia to the water every day, encouraging bacteria to grow, and once ammonia is down to zero you add just enough more ammonia to keep it going and therefore nitrite-eating bacteria grows, and then you can add fish. And you shouldn’t lose any to the ammonia or nitrite, which would be nice.

We got too many fish, too fast. I’m pretty sad about it.