It is a fairly technical article, to be sure, but bacterial blooms are so common in new tanks I think it well worth trying to get an understanding of them.
You say it was a used tank ? Did you give it a thorough cleaning before you filled it ? Did you use new filter media, or was their old filter media in the filter ? The tank wall or filter media that had dried out could easily have been sources of large numbers of heterotrophs. Incidentally, heterotrophs are not all bad. They form biofilm, which is really quite essential to have in a tank. It's a food source for many creatures, from fish to shrimp to snails, and is normally harmless enough. It's only when it has this overabundance of food sources and no competition to speak of that their numbers explode and cloud the water.
I see no need for meds of any kind. I can't even think of any that would apply.
Measures you can take to improve the water include the following:
Activated charcoal. If you don't have it in the filter, get some. Add a good handful or two where the filter media go. Mesh bags are good to keep it in one place. You can get filter pads with activated charcoal in them you can cut to fit if that's easier. Use the charcoal for two weeks, by which time it will be exhausted and need to be replaced. If you leave it for longer than that it will begin to discharge all the organics and toxins it has collected back into the water. This is the reason charcoal must be changed often, and isn't usually needed except in certain instances, like this one.
If the bloom has gone when the two weeks has passed, no more charcoal is needed. But if the bloom has not gone, or it comes back, add fresh charcoal for another two weeks. Once the bloom is gone, remove charcoal, and replace with some other biomedia instead. Floss, sponge or ceramic, whatever you choose, all will grow BB.
Charcoal is usually only used to remove meds, tannins or other colours, and other undesired organics from water, like bad odours. In this instance it's removing what the bloom is feeding on.
Because you don't have fish, it does not matter if the bloom should happen to increase your ammonia level. If that happens, it will be temporary anyway and the BB would deal with it eventually. It's only a problem if a spike like that occurs when fish are present, in which case you have to do as many emergency water changes as are needed to get the ammonia down to no more than .25 ppm, if not to zero.
Increase water turnover. This will increase gas exchange. Where there's long lasting or large bloom, heterotrophic bacteria may reach high enough numbers that they up too much oxygen from the water before it reaches the BB in the filter. Those BB need a steady supply of oxygen, so you have to make sure they get it, before growth can seriously slow down, maybe even stop.
If you have an air pump and air stone, use that. You can also lower the tank's water level a bit so water returning to the tank from the filter falls a few inches. The splashing increases surface area, allowing greater oxygen exchange into the water as well. Do both if you are able, more is always better where oxygen is concerned in this sort of event. Never hurts to review this article periodically, just to remind ME of all the various things that can be affected by a bloom
In this case also, I would lower the water temperature to 22 - 25 C, until the bloom has gone and water is clear. The cooler water is, the more oxygen it can naturally hold. The warmer it is, the less it can hold. You can raise the temp back up once the bloom clears.
If you've been adding any sort of fish food, stop doing it and gravel vacuum to remove all such organic waste because it also is feeding the bloom.
Last, some extra water changes will assist to cut down the numbers of bloom bacteria. Water changes all by themselves don't work but in combination with the other measures, they'll help clear the water sooner.
Started fishless cycling it last week. I just turned the filter/pump/heater all on and assumed that as it was second hand it would go to roughly the same temp as previous owner had it at. The next day i checked it and the tank was at 38 degrees and had gone cloudy from being crystal. Is this bad??
