We've just set up a tropical tank again after about 30 years!
We had one years ago, but when we moved house gave the fish away, as it was along journey and we were worried about them. Then the kids won a couple of goldfish at the fair so the tank got used for them - it was a 48" x 15" x 18" tank, and the fish outgrew it. They had to be moved into a pond - and then DH dropped the tank moving it and the glass cracked.
So we have had a number of fishless years until he declared he wanted some again.
We got this tank"
www.petsathome.com/shop/en/pets/fish-tanks-and-cabinets/love-fish-panorama-tank-64-litre-%28in-store%29
And it's a very attractive tank - the only complaints I would make are that I don't think the filter is powerful enough (it's not dreadful, but it could be better) and there is a light which is supposed to tell you when the filter needs changing, but it hasn't yet. I'm pretty sure you can get a stand for it if you want one - we've just got ours on a low TV unit as we hadn't really got room for a stand, but they can be handy as you can keep all of your fish paraphernalia in the cupboard. It's a matter of preference really, but you need to be able to ensure that the tank is level. Pets at home will give free delivery on expensive items like this. I don't really know any online retailers to recommend them, I'm afraid. The one we go to locally isn't a big firm and doesn't deliver.
We did our fishless cycle (took a little over a month) and got half a dozen glowlights about three months ago. Have to admit we got them from Pets at Home just because we got a "five quid's worth of fish free" coupon when we got the tank, but I'd recommend that if there is a specialist aquarist shop you can go to, that you give them your custom -
Pets at Home aren't the best for stock or advice.
We got this water testing kit
www.amazon.co.uk/API-Freshwater-Aquarium-Water-Master/dp/B000255NCI/ref=sr_1_5?dchild=1&crid=3B68G4FNCZSO9&keywords=water+testing+kit+for+aquarium&sprefix=water+testing+kit+for+%2Caps%2C148&tag=mumsnetforu03-21&qid=1590071462&sr=8-5
It's expensive but will last a long time. DH tests the water every fortnight.
At present we have the 12 tetras in - I'm not sure how many more we can comfortably accommodate (that's what I wanted advice on). I'm hoping perhaps another half dozen. And I would also have like an algae eater - ideally I'd like otocinclus catfish - they are delight - but they really need to be kept in shoals of 5-6 minimum, and I don't thinkour tank will be big enough.
TBH - I wish, in retrospect, that we'd gone for a bigger tank, but this was DH's hobby and I didn't realise I would get sucked into it all over again (I couldn't help myself), but having said that, this tank is nice size for our living room. It's big enough to enjoy looking at, but not so large that it dominates the room. The filter is quiet, and it has LED lighting which has the advantage of not getting hot the way the old bulb-type lights used to. And the glass is shaped on the front and sides as one piece, so there is nothing unsightly in your eyeline when you watch the fish. The top is excellent quality - many tank hoods are very bendy and fragile., but this isn't, and the feeding flap is well-placed, too.
I think that if you want the tank on your unit this would fit (it's about 60 cm long, by 35cm (front-to back) x 40 cm tall. It isn't a bad tank at all, but there are a lot of really nice styles of tank available - could you get your dH a voucher from an aquarist's and let I'm pick his own? (I don't know if they do this, but you never know).
Sorry I can't be more help. Let us know how you get on.