Can I use a old marine tank for tropical fish?

[Rootvegetables]

Just that really, it has a filter, heater and some sponges and a bag of carbon. Anything else I should change or get. Anything else I need to consider?

It's an Orca tank if that helps anyone!

Sure, should be fine if you give it a good scrub. I'd bin the carbon (freshwater tanks don't need it anyway) and the sponges, but the filter, heater and tank should be fine if you give it all a really good clean. I'd probably use some white vinegar and then rinse it loads of times.

How big is the tank? Have you got any species in mind? Once it's all set up and running, you'll need to do a fishless cycle before adding any fish.

Thanks for your help! It's 100 litres so I think just some little ones, maybe some guppies, any advice on what goes well with them or maybe a single more exciting fish that would live happily with some little ones

Ooh, 100 litres is a lovely size, lots of options without being too big. A lot depends on your water, if it's especially acidic or alkaline then it will rule out a few species. You could have a shoal of small bottom feeders, a shoal of small mid-water swimmers and a pair of small centrepiece fish.

If it were me with my water (pH7, med/hard) then I'd probably have something like-
6 or 7 panda corys
6 or 7 small tetras, probably flame tetras or good old Neons
A pair of dwarf cockatoo cichlids (make sure you don't get two males!)

If you decide to have multiple species then you'll need to build up the stock gradually and you'll also need a quarantine tank. I'd also think about building up the shoals of corys and tetras over time if the filter was up to it.

Personally I'd avoid guppies, they are so inbred now that they're not as tough as they used to be. Endlers are a better option now if you want a small, bright fish that will do well in hard water. If you have males and females then you WILL end up with loads of babies!

I'd like babies! I will investigate endlers! Centerpiece fush is a good turn of phrase and I like cichlids, so it's a possibility certainly. It's all up and running now and doing it's cycling business, at what point can I add live plant?

You can add live plants whenever you want. They won't affect the cycle and if you add them sooner rather than later that'll give them a chance to settle in before you add fish.

Do you know what sort of water you have?

I'm not sure on water type how would I find out? I'm in se London. Also does the type of lighting matter to the fish as the aquariums original lighting is still working will tropical fish mind marine lighting? Is it even different?! Thanks do much for all your help.

There are different types of lightbulb with different spectrums for plants, coral etc but it's not that different. Marine lighting tends to be a lot stronger. The bulb should tell you whether it's a T5 or a T8.

In SE London you've probably got hard water- clues are limescale in the kettle and a bathroom that's a pain in the arse to clean

Okay another question, hope I'm not boring you! I added my first fish yesterday, 2 peppered Cory's and a few danios but my filter is so strong I'm not sure if it's too much, the Cory's seem okay on the bottom but the danios seem to be working quite hard. The inlets at the top right it's currently above the water to try and reduce some of the pressure. Do fish mind or should I try and reduce it somehow? If so how? Thanks!

Danios shouldn't mind a fast flow of water- have you checked the water quality? I'd keep an eye on it, the tank won't have cycled by now so look out for the ammonia and nitrIte levels.