My coldwater marine aquarium.

Its been a good few months now that I have been messing around with keeping a coldwater marine fish tank. My fiance used to keep a couple freshwater tropical fish in it but after they died and the move to our house in Staffin, the tank was just lying about in the shed unused.
It was then when on my LRF sessions, I was getting loads of small fish from the piers and I decided to have a go and utilise it.
I have to admit that there were failures (and loss of life 😞) but I think that I have now got the hang of it. Below I'll give some details on the tank and gear that's worked for me, along with pics of the inhabitants.
The tank is 120ish litre which is about the minimum recommended for saltwater fish tanks. The filter is the Aqua-Flow XL  but also a Hidom AP-2000F 1600LPH for back up.
I've got a Vlike air pump in the tank for proper oxygenation.
After buying an appropriate protein skimmer for tanks to 150L. It became apparent that even though it was working well at first, the amount of scum collected from my crowded tank in the collection cup was needing several changes throughout the day (tedious). That protein skimmer also malfunctioned after a couple months as the water intake (nozzle) had no filter and various bits of food got through and stopped the little fan/prop from working.
I decided to get a new protein skimmer, much larger this time and meant for tanks up to 600l! Its the DG2516 by Boyu and so far it's doing a good job. It needs a bit of adjusting and was skimming like mad at first but once I found the right setting, it stabilised!

Livestock and some notes.
At the moment the following are in my tank:
1 pollock
2 black gobies.
2 rock gobies
1 or 2 butterfish (shy!)
4 sea scorpions (awesome!)
1 flounder
2 blennies
1 limpet
4-5 hermit crabs
1 wee brown crab
few anemones
unknown number of prawns and winkles along with copepods/zooplankton.
I'm mainly feeding them frozen whitefish, scallop offcuts, prawns along with some dried Artemia pellets. Clear preference to the frozen food as expected!
I'm feeding twice a day and usually enough food to be eaten in a few minutes from the fish (limiting ammonia issues as much as I can).
Some further things to consider:

• Rockpool species the easiest to keep.
• Free-swimming species more sensitive to oxygen levels etc. But also increase strain on the whole system (more food, more oxygen, more poo!)
• Some species far better suited for fish tanks than others, ie pollock and whiting are pretty chilled, codling and poor cod so-so, coalfish not suited at all (maybe need more space).
• Some crabs (eg velvet) really thrive and can become bullies when larger.
• Hermit crabs and prawns are good cleaners and also act as extra food for the other fish...
• Don't add seaweed/kelp in the tank unless you know what you're doing!
• Living close to the sea is handy as you can collect seawater easily and thus prevent blooms, ammonia spikes and the like. Also, it can get very addictive collecting new 'tenants'!
• A bucket of fresh sea water every week along with gravel-hoovering, help immensely in keeping a healthy environment.
• After returning from holidays and turning on the heating in the house plus cleaning the filter, was when I'd get bad blooms. Best to partially clean filter with some tank water...

At the moment, I have my fish tank in the hallway, where its always well below 18C and this means I don't need to add a water chiller. Hopefully, it will be ok in the Summer too.
The bottom of the tank is gravel, though I think a couple inches of sand would be better (to help worms and creepy-crawlies hide).
Some pics below.

A not so great pic of Papi the black goby...

One of the sea scorpions when first in the tank.

Sea scorpion tree...

Paulie the pollock.

The smaller black goby.

My wee flounder, a mostly nocturnal hunter.

A spirited blenny.

One of the rock gobies.

Blenny and sea scorpion.

Spot the butterfish...

New favourite tree for the sea scorpions.

Hello.

Big prawn..

Another goby..

Hermit crab and sea scorpion.

Paulie getting a big belly.

Camouflage..

Even better camouflage!

I have to say that the fish tank has provided us with hours of entertainment, not just by observing the creatures but also collecting them! It can seem disconcerting to others, watching two adults rummaging around rockpools with little nets and buckets, but we couldn't care less!
Especially now, in the middle of Winter with frequent storms and unfavourable conditions for fishing, keeping the fish tank somehow improves my mood and I don't feel that bad when I cant go out.
Plan for the warmer months, is to get a larger/longer fish tank so that fish will have more space to move and also try and add a couple wrasse of some kind or even other more unusual little fish.
It's very intriguing watching them and seeing the behaviours each species exhibit. So far I have to say that my favourite fish are the sea scorpions! They show little fear and can be very aggressive, even grabbing bigger prawns than the pollock!