18-20L is really too small for fish, sorry There just isn't space to swim or form territories and the water volume is too low to keep stable and safe with fish in there. Usually tanks that small are used for shrimps or snails and so on.

Have a look at the care sheets linked on the left and you'll see each sheet gives the minimum recommended tank size for the species. 60 litres is the bare minimum for platys.

If you genuinely have a female the males will continue this on and off until her eggs are released. If they've confused another boy for a girl (the round shape confuses them sometimes) it may go on for a lot longer and eventually subside until the next breeding season (spring time). Have a look at the presumed males for little white bumps around the gill plates - these are breeding tubercles and appear on the males in spawning season. As the season concludes, they will fade again until next time.

When looking at shoaling or schooling fish usually 6 is the minimum, but in some cases more are needed. This is the case with Zebra Danios who can be nippy and aggressive towards other fish and each other in small groups.

Zebra Danios aren't truly tropical; they need the temperature to be below 24C which is on the cool side for your other choices there. If you're keen on keeping them then you'd need to look at other temperate species' who do well at lower temperature, rather than tropicals like the Black Phantom Tetra.

Regarding water hardness - the Black Phantom may cope at that level of hardness, but in common with most of the Tetras that have a degree of tolerance for harder water, it is likely to become pale and not look its best. They do far better further down their range of tolerance to be honest.

You may have problems with algae in that position. Does the window get direct sun at any time of the day? I know most do, but I have a window that faces directly onto the next house and also faces east, so although it's breaking the rule, my largest tank does face that window, but the lack of any direct sunlight means it causes no problems.

Hi and welcome,

It sounds like you have two males and a female (or at least one the others believe is female) and they were trying to persuade her to spawn. spawning behaviour is often triggered by a water change.

It would be a very good idea to look at getting a bigger tank as that will give her more space to escape and reduce the danger of damage from collisions in such a small space. 60 litres is really much too small for 3 Goldies long term and the round shape is not good for swimming space or for good oxygenation as the surface area is very small. This article goes into the size of tank they need and why they need such large quarters.

Sounds like cotton mouth - Flexibacter columnaris. This can be nasty and hard to treat unless it's nipped in the bud at the first sign of trouble.

What are your current tank readings for ammonia, nitrite, nitrate, pH, gH and temperature? What size is this tank and what fish do you have in it (a list of all speicies' in the tank and number of each)?

This disease is usually brought about by stress, water quality issues, incorrect parameters. It is highly infectious so you should treat the whole tank rather than individual fish and use separate nets, buckets and other equipment if you have other tanks to avoid any cross-contamination.

When this disease is established (as in this case) the best chance of a cure is with antibiotics, but they are vet prescription only in the UK and the vet will want to see the fish. You may have success with eSHa 2000 or Myxazin if you don't want to involve a vet, but before you start you need to do a large water change, gravel vac and filter clean to bring the water quality as close to perfect as possible and remove all other meds and salt.

Hi Splash,

I usually use airline to syphon shrimp tanks

Oh dear! Not fun.

I think your best course of action is to drain the tank, refill, run for 48 hours then drain and fill again. While that's happening get a bottle of this which is pure ammonium hydroxide and the right strength (I use this brand from that eBay seller myself), then start the cycle again.

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Measure the tank's length , width and height and use the calculator on the right of the page to find the volume in litres. Then get a heater equivalent to 1 watt per litre. If that's an odd size, go for the next size up.

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Go for either an internal filter or an external canister as you prefer. Externals have more volume for media and have the advantage that they don't take up space in the tank. Using the volume you worked out earlier, look for a filter that can turn that volume of water over 4 times an hour. So for a 100 litre tank go for a 400 lph filter.

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Many of the specialised planting substrates are designed to be used below sand or gravel so you may need both. However, whether you need it or not is partly dependent on what kind of plants you intend to have. Bear in mind that if you use a planting medium to provide high fertilisation you need plants that can use it and possibly high output lighting and/or CO2 injection to enable them to use it. If water contains many nutrients but the balance between nutrients, plants, CO2 and lighting isn't optimised, the nutrients won't be taken up by plants and will give algae a chance to take hold.

It's possible to to be densely planted and still have a low tech tank if you choose plants carefully. Plants Alive show all the requirements of their plants on the web site so is a good place to look. There's a discount voucher for FK members in the members' lounge.

Before choosing plants or fish, find out what the pH, gH, kH and nitrate levels in your supply are as these directly influence which species' can be kept.

You only need to do it once and if it's short, measure how much extra you have to add to get it to 4ppm. That will give you a way to calculate the actual strength for future doses.

Once you get to the stage where you can dose 4ppm ammonia and 12 hours later have 0 readings on both ammonia and nitrite your cycle is complete. Then you need a big water change to reduce nitrates and replenish minerals before adding fish.